Facebook's $1 billion acquisition of Instagram is paying off as the social network taps the photo-sharing application's expertise in building mobile engagement on a shoestring. Still, founder Kevin Systrom says he'll soon have to find ways to pay his way by monetizing the service without alienating its 100 million users. "It's like living at home after college. There comes a time when you have to add to the pie. If Instagram is going to become what I want it to become, then it has to do that. It's gotta grow up," he says.