Foursquare is a cross between a social networking app and a geolocation game.
On the one hand, it allows you to share your location with friends so they can see some of the places you visit, but on the other hand it’s a game where you repeatedly check in at a location - say a coffee shop - to earn points and badges. You start off at Newbie level, but can work your way up to being Mayor of various locations.
Of course, once you become Mayor other people will try to knock you off the top spot by racking up more points than you. The Android version differs from the iPhone app in that it integrates with Google Maps and has a one-click check-in process.
Foursquare won’t be everyone’s cup of tea, but this is a great app for those already addicted.