At today’s Mobile First CrunchUp, Erick Schonfeld sat down with AdMob and Churn Labs founder Omar Hamoui, and Instagram founder Kevin Systrom to discuss how both founders approached mobile development vs. the web.

Systrom, whose popular iOS photo sharing app just passed 7 million users, explained that the biggest opportunities for Instagram are on mobile, not the web. “I don’t believe Instagram could have been a website and website only,” Systrom explains. “The nature of the app is about sharing photographs in the moment.” And the mobile phone is the optimal platform for this task.

Of course, Instagram is growing fast for an iOS only app, adding 2 million users in less than two months. While Systrom believes the future for Instagram lies in mobile development (hopefully that means an Android app soon?), there is work being done on a stronger web presence. But he says it’s not as simple as just putting all the iPhone app’s functionality on the web. He explains, “We’re trying to create what we have on the iPhone and make it an interesting experience on the web.”

UPDATE: In the final panel at the CrunchUp, Systrom did say that and Android app will be launching in the future. But no timeline yet.