Foursquare works in conjunction with social networking services and allows users to "check in" at restaurants, stores, landmarks, movie theaters, etc, from their mobile phone. It taps into the phone's built-in location data and then broadcasts that information for the world to see. That is, the part of the world that users have granted access to. Only friends are allowed to see where users have checked in.
Recent online polling suggests that users are becoming less and less concerned with invasion of privacy online, and are more willing to share personal data. Younger consumers are even okay with letting mobile advertisers tap into their location information in order to provide relevant ads.
Foursquare has come under the microscope lately as social networking juggernaut Facebook has launched its own location-based service, Facebook Places. It's not as feature-filled as Foursquare, though, which leaves Foursquare's founder confident that it can remain relevant.
The flagship location-tracking service reaches three million users just a month and a half after it hit the two million user mark.