Foursquare is the leader in a new generation of "find a friend" applications. According to their website, Foursquare "is a cross between a friend-finder, a social city-guide and a game that rewards you for doing interesting things."
To use Foursquare you simply tell the application where you are and comment in some form. This is called a "check-in." Foursquare targets mobile phone users and the vast majority of check-ins are done using smartphones.
Below is an example "check-in" that I grabbed off the Foursquare feed:
Graham W. in London, United Kingdom:wrote a tip @ Thai Aubergine: Try the chef's special Salmon. It was awesome.