Breakout Companies of 2010

AP Images
4 of 11

Foursquare
New York City
Founded in 2009

Why they’re on the list: Because they’ve added 4 million registered users (and counting)

With the advent of the iPhone, and the onslaught of so-called “smart phones” that followed, it was only a matter of time before companies found a way to get customers to use their phones for more than just playing Tetris. Then came Foursquare, the company that now enables some 5 million users around the world to use their phones to “check-in” at, say, their local coffee shop, and then notify their friends of their location via Facebook and Twitter. The application also awards “badges,” a kind of online plaque, to users for frequent and unusual check-ins, which has made Foursquare not only addictive, but also wildly successful. The company recently passed the 100 million mark for total number of check-ins to its site.