Foursquare, the location-based social network which launched in London recently, has rolled out its service in fifteen more cities, adding to the 21 already available to bring the total up to 36.

The cities added are all in the USA and Canada, and the full list runs: Baltimore, Cleveland, Indianapolis, Kansas City, Milwaukee, New Orleans, Omaha, Pittsburgh, Raleigh-Durham, Richmond, Salt Lake City, San Antonio, St. Louis, Montreal and Toronto.

Foursquare challenges its users to "check in" at locations using GPS, describing itself as 50% friend-finder, 30% social cityguide and 20% nightlife game. The idea is that you discover new things while being aware of where your friends are. It comes as an app for the iPhone and Android platforms and a mobile site for other smartphones.

If you'd like to find out more, Pocket-lint has a guide to Foursquare, as well as a set of Foursquare tips from an addict.