The must-have app on the iPhone is not iMessage. It’s not iTunes or Safari or even Find My iPhone. It’s Instagram, the photo-sharing app that once described itself as “quirky.” Cultish would be more appropriate these days.

Those who use and love Instagram are locked to their iPhones in far tighter fashion than any contract could ever manage. Just ask someone whose Instagrams show up regularly in your Facebook or Twitter feed; they’ll tell you they’re incapable of using a phone that doesn’t have the service. Yes, Instagram is where your friends drop photos of their lunch. But it’s more than that: It’s a real-time window that peers out into the wide world. It’s where millions post photos of revolutions, riots, the ugly, the beautiful, and the banal. Abandon the iPhone and that window slams shut. And that’s a powerful incentive to stay put.

Sure, there are other notable iOS-only apps—Instapaper, for example, or Flipboard. And of course Apple itself makes a slew of useful apps that best anything similar on competing platforms—like the aforementioned iMessage and Safari. But all of those have analogues on other platforms. Not Instagram. Others apps may duplicate what it does—when it comes to photo processing, that is—but without the network, none have its appeal. You want to be on the thing your friends are on. You want to go where the real-time visual network already exists. And, for now at least, that’s not Android or Windows Phone.

In retrospect, Instagram is obvious. Take a picture. Make it better. Share it instantly. That’s all it does, but it does it very well. The app applies photo-filters that add a veneer of fake nostalgia to the present day, with Land Camera and Kodachrome hues. Then, if you want, you can post it on your social network of choice.

You can think of it like a visual Twitter, or a quicker Flickr. But mostly, you should think of it as a bonafide blockbuster.

Instagram launched on October 6, 2010 at 12:30 am, with just two people: company founders Kevin Systrom and Mike Krieger. Both very young: 28 and 25 respectively. Kevin has a lot of presence. He’s tall and direct and on point; in person he’s a lot like he appears in Best Buy’s recent Super Bowl commercial. He had been an intern at Odeo where he met Jack Dorsey and Ev Williams, and watched Twitter take off. Then, while in grad school at Stanford, he met Mike Krieger, an affable Brazilian programmer with an easy smile who still comes across as completely awestruck that he’s got this wolf by the ears. Together, they founded Burbn, a location-sharing service. It failed to catch fire. But their second try? Five alarm.

There are a mere ten employees at Instagram—only eight of whom are even in the US. Yet in the past year and change it has racked up more than 15 million users, who have uploaded some 500 million photos. The service has pulled in $7.5 million in investments, including money from Twitter founder Jack Dorsey. It’s already available in ten different languages, and, having already scored a hit stateside, is piling on users in China, Brazil, and throughout Europe. All this without having a Web interface. All this while only being available on the iPhone. And all this while iterating and scaling and building out the back end like a company that commands an army of developers. It doesn’t.

Based on outward appearances, you might think the Instagram crew a relatively quiet bunch—after all, they only have an iPhone app and a placeholder-like website that offers basically no functionality. But since last October, the team has added hashtags and autocomplete functions, completely reworked the comment interface, added email sharing, converted the entire image pipeline to Open GL, created a news view, added support for high res images, tacked on 10 languages, and rolled out a slew of new filters and other features. All this while completely rebuilding the back end to support its exponential growth. Phew.

And despite all that, the only thing people want to know is when it’s coming to Android, or when it’s coming to Windows Phone. We do too. Because when it does? 15 million will seem like a blip.

Justin Bieber Is a Scaling Problem

Instagram isn’t just small; it’s tiny. It’s miniscule. It is famously located in Twitter’s old digs in San Francisco’s South Park neighborhood. But here’s the thing: Instagram subleases its space from another company. Instagram isn’t in Twitter’s old office, it’s in Twitter’s old conference room. The entire company is nothing more than a collection of desks arranged bullpen-style in a room that is smaller than most two-car garages. There’s also a small reception area (sans receptionist), a collection of vintage cameras, a cow rug and, well, that’s it. But for a little company, it has some very big names.

“Celebrities cause things to break,” explains Systrom, matter-of-fact, while slurping down a coffee approximately the size of a cow heart.

Just the day before, President Barack Obama had signed on and begun sending out photos. This seemed like a real sign that Instagram had arrived. Obama already has accounts on Flickr and Facebook. He (or his people) must have seen something unique and wonderful in Instagram’s audience, some way to reach people via that channel that it couldn’t through others. When the President joins your network, it’s news. And while it’s great news, it can be the kind of thing a company isn’t prepared for. But as it turns out, Obama is a fractional compared to Justin Bieber.

“Our first celebrity was Snoop Dogg. I remember getting an email that was like ‘Snoop Dogg’s people want to talk,'” says Systrom while Mike chuckles. (Mike chuckles a lot.) “That was a year ago. It’s amazing how we’ve grown as a company since then. I think for us to say that, when we set out we expected it would take a little more than a year for somebody like Barack Obama to sign up, that would be a lie.”

Nonetheless, today Instagram is lousy with famous users. Snoop, Obama, and Bieber; Tony Hawk, Jamie Oliver, and Ryan Seacrest are all onboard. So too is the media. National Geographic, NBC News, ABC News—even Playboy has an account. All those heavily-followed accounts have done more than give the company bragging rights, however. They’re edge cases that have helped Instagram build for the center.

“We learned really quickly, especially with celebrity users, about the design and what would break,” explains Krieger, who leads development as the company’s CTO. “We didn’t really design with 50,000 likes in mind, but now suddenly this Bieber photo has 50,000 likes. That is a totally different UI issue. Or all the sudden he has hundreds of comments, and you can’t display them all at once or the app just scrolls on endlessly.”

It was because of Bieber and other popular users—including ones who have made a name for themselves on Instagram, like photographer Cole Rise—that it started comment threading and made the meta-information below a photo collapsable. Otherwise, the app would waste valuable screen real-estate when users simply wanted to get to the next photo in their stream. And it hints at why it isn’t on Android, or Windows phone, or, God forbid, Blackberry yet: They’ve been scaling.

What I Saw at the Fail Whale

Twitter was nearly undone by scaling problems. Just as it began to really take off, in 2008 as it got into the millions of users territory, it began breaking down with regularity. The Fail Whale was everyone’s least favorite mammal. And that was almost entirely due to Twitter’s back-end, which was originally designed as an extremely simple program that ran on an open-source application framework. As it turned out, that didn’t scale.

Twitter’s architecture was fine for content management—like a blogging platform—but not so great for a true real-time communications system with an exponentially growing user base. Messaging systems have specific needs—reliable queuing, concurrency, robust caching—that it just didn’t have. The Twitter team tried to write those features themselves and attach them to the existing framework, but every addition created even more stability issues. One of the biggest issues was the simple growth of the size of people’s networks. Rails, around which Twitter was built, was designed to simplify complex database relationships by making many queries. But the number of queries needed to support the complicated social relationships people build on Twitter were overwhelming its servers. The end result? Very many people who were very frustrated with Twitter.

Systrom saw all this from up close at first, and then at a distance. He was an intern at Odeo when that company cratered, and from its ashes rose Twitter. All of which helps explain why Instagram is not on Android yet: the team has been doing so much work on the network’s back end to make sure it doesn’t suffer similar problems.

“The best feature is that it works” explains Systrom. “You compare our history to other social media startups and it’s been very good. We’ve been very careful about scaling.”

And to that end the team has spent much of the last year planning for growth. Because they don’t want to revamp the system while its under load, that’s meant doing things like calculating where likes-per-second will be in a month—or six months—and reconfiguring the app and back end to support it. There is no Fail Whale or Tumblrbeast of Instagram. There’s just uptime. And they want to keep it that way, even as they continue to blow up.

“To be honest, 15 million is a small fraction of where we want to be, so we have to think 6 months to a year ahead,” says Systrom.

That’s both complicated and amplified by the fact that Instagram is mobile only. It’s not a web app. It doesn’t run in the browser. So to get to that next level, it has to support completely different platforms. And doing that means offering a user experience on Android that’s the equal of the one on iOS. And that’s a big problem to tackle, made all the more so by the expectations game.

More than a year ago, Systrom announced that Instagram was coming to Android. So. Where is it? That’s all everyone he talks to wants to know. Systrom is outgoing and direct, but he’s so prone to facetiousness that it’s not always clear when he’s being serious. It can leave you feeling uncomfortable, wondering if he’s fucking with you. When I asked about Android, he came across as being wearied by the question. In fact, he seemed almost resentful. But it was hard to penetrate that mask of sarcasm.

“You guys have been talking about Android phones for a while now,” I began.

“For a year!” He interjects. “And I’m gonna keep talking about it!”

“To be at 15 million users on one platform is not something any other social mobile company can say. There are a lot of others that are in that size range but are multi-platform. We saw an opportunity to be really good at one thing, and it turns out that helped us. It wasn’t because we felt like Android wasn’t an opportunity we wanted to go after. It wasn’t about the quality of the phone—there are plenty of awesome Android phones; we have a bunch in the office that have beautiful displays and beautiful cameras. It’s more that we were three people trying to keep the site up. We’re now eight people, with ten people worldwide.”

“The only thing that will make other platforms happen is natural growth of the team. I think we didn’t expect how quickly we’d grow a year ago when we were like ‘oh let’s work on Android next.’ Everything became a priority, and because everything became a priority we had to focus on what was most important which was to keep the site going and make users really happy. A person is a person is a person no matter what phone you own. I’m excited to be on Android someday. Are you kidding me? Our growth is going to double.”

And as to those rumors that Instagram is going to develop an app for Windows phone before Android, leap-frogging the most popular mobile platform in the world? “We’re evaluating all different platforms all the time, but Android is very much our obvious next step,” he says.

And if that’s an obvious next step, there’s an equally obvious way to get there. For a company valued at $20 million, with almost a million dollars invested for each employee, the big question is why they haven’t gone out and hired more people.

“We only hire the best of the best.”

It’s an unlikely answer, puzzling even. But that small team, of course, also helps keep them from burning through cash. More people mean more paychecks, which means more of a need to generate revenue. And with its big stack of cash, it has the luxury of taking its time. At least, for now.

Beyond Facebook

“When we launched [the iPhone 4] was like three weeks old. It has this great camera and this beautiful display. There were a lot of people who had tried to do what we had done before. We tried to be different in a few key ways, but one of them was the timing,” says Systrom.

And indeed, timing is everything. If past is prologue, Instagram’s time is about to come in a bigger way that it ever has before. Because Systrom and Krieger exude ambition. Instagram doesn’t just want to be bigger than, say, a competitor like Path. It wants to be Facebook big. It wants to become the indispensable visual strorytelling medium. It wants to be an entertainment platform, where people can come and consume. And getting there means getting on every platform, in every country.

“What interests us are the natural limits in terms of how many people are owning these phones, whether that’s Apple, or Android or whatever it is. I don’t foresee a future where people don’t have some sort of phone that’s like a computer. I don’t foresee a future where those phones don’t have cameras in them. That spells a future where smartphones are the status quo. You have to ask yourself how you allow people to communicate what’s in their lives,” says Systrom. “I don’t like the idea of Instagram as a photo sharing service, and I don’t think it is,” says Systrom, “it’s very much a communication tool, it’s a visual communications tool.”

“The printing press did something really big for the world when everyone could get books in their hands and read. I’m not saying we’re a printing press, but I am saying technology pushes people forward in some way and unlocks potential. we’re not focused on how we can make toys, we’re focused on how do we change the world in some real way. Like, how many companies have been handed the opportunity to get 15 million users in the first year? Not many. We want to take this ticket and ride.”

And so Instagram plods along. Deliberately. Methodically. Not caring what you think about its pace. And you know what? If Facebook doesn’t eat it. If Twitter’s new hosted photos, or some other competitor, doesn’t destroy it. If it can finally ship an Android app, and then a Windows Phone app. If it can just keep doing what it’s been doing, but bigger, faster, better. If it can do all that, it just may get there. Either way, it’s going to be fun to watch.