after interning at the company that would become Twitter, he always had the itch to do something on his own.

Back then, location was hot, hot, hot. Foursquare had launched about a year earlier and had made the check-in a cultural phenomenon. It made sense to experiment with location. Perhaps an HTML5-based check-in app was the way to go.

Users weren’t exactly checking in all the time on Burbn. Instead, we were sharing photos.

The photography category seemed saturated, but Systrom saw an opening that many others didn’t. Hipstamatic was super-fun and came packaged with all of these filters, but it wasn’t very social.

With their UX skills, Krieger and Systrom refined Instagram to require as few actions as possible. Unlike the original version of Path, Instagram didn’t force users to add tags about people or places to their photos.

Mirroring Twitter, they made Instagram public by default.

Instagram hit one million users in three months. Then that became two million, which then became 10 million users. Unlike many apps at the top of the charts, Instagram didn’t have to spend a dime to get where it was. It was organic growth.

Every month seemed more unreal. When Justin Bieber joined, there was literally a visible spike in activity as thousands of girls responded to his every photo.

All the while, Systrom kept saying he never felt threatened by Facebook.

Instagram kept its app lean. They didn’t change much to the app’s essential experience even as its user base ballooned. It was more important to say ‘No’ to new features instead of ‘Yes.’

Indeed, when you have a consumer-facing app that is going to be advertising dependent, scale matters most. If you don’t have the eyeballs, you don’t get the advertising dollars.