The story of Instagram is insane. Only two years ago, it was an idea. That idea sounded good enough to land Kevin Systrom and Mike Krieger $500,000 to launch what was then called Burbn, but it was still only an idea. Eight months later, Instagram went live in the iTunes App Store and its user base has been exploding ever since.

By August 2011, the fact that Instagram had been used to publish 150,000 photos was considered big news. Not even a year later, that number pales in comparison to the one now grabbing headlines: 1 billion. That's how many dollars Facebook spent to acquire the mobile photo-sharing app, after only 17 months of its existence.

Last week, Instagram launched its long-awaited Android app and racked up a million new users in a single day, just as it had done with its original iOS launch. This week, the Facebook acquisition landed Instagram on the front page of The New York Times and at the top of the App Store's free apps list.

Despite the knee-jerk freakout some users are having about the Facebook connection, Instagram is barreling toward the 40 million user mark and its growth doesn't appear likely to slow anytime soon.

Exactly what Facebook will do with Instagram is yet to be seen, but Zuckerberg has promised that the app that millions of people have grown to love won't be dismantled. Instead, the acquisition will likely help Facebook monetize its mobile efforts moving forward. It also makes perfect sense when you consider the value that photo sharing has for Facebook as a whole.

This is pretty much the wet dream of just about any tech startup. Launch a product, have it garner insane levels of popularity and then sell it to an established giant for a cool billion. The Instagram story is intriguing because seldom do these events actually take place on this scale, this quickly.

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Infographic courtesy of Visual.ly.