Age: 33
Occupation: Co-founder, Foursquare
Previous TIME 100 Appearances: 0
In 2009, we tweeted. In 2010, we'll "check in." Foursquare, which Crowley and Naveen Selvadurai launched last year, is at the forefront of a new generation of location-based social-networking services. While sites like Facebook and Twitter are interested in what you're doing, Foursquare is more interested in where you're doing it: with an iPhone or other mobile device, users in more than 100 cities can check in to let their friends know where they are. Check in enough times at a given place (a coffee shop, say, or Hooters), and you can earn special bonuses. That gaming component is the site's secret sauce — one that has pushed it past 750,000 users.