Alex Rainert

When Alex Rainert joined Foursquare in March 2010, its user base was 500,000--and people were mostly just checking in places and earning badges. Now that number is nearly 40 times larger, on account of Rainert utilizing what those first 500,000 provided. "We have so much rich data about the time-specific connection between people and places; we can give people real-time content that reflects those three axes," he says. New services include tracking user check-ins to offer honed, personal recommendations on where to go, allowing users to search for new places based on their own history and others' recommendations, and letting musicians reward fans if, say, enough check in at a concert. It's part of a grander vision, he says, which isn't just about places: "It's a way to keep track of all the things you want to do."