A rising social media site is foursquare, https://foursquare.com, which is city based and works on all possible mobile devices.

 This site combines two rising markets, local and mobile, with social media very cleverly. Users of the site are encouraged to physically (a nice change!) explore their city with locations shown on their device shared via the site.

The Mayor of New York City, Mike Bloomberg, recently announced that April 16th (4/16) would be 'Foursquare Day' as the web start-up is run out of NYC.