What is Foursquare?
Primarily a “location-based social networking service” for smartphones and mobile users. Users “check in” to places they’re at so that their friends know where they are. The most frequent visitor of a place (bar, restaurant, venue) can become the “mayor”, and users can also earn badges for completing special tasks.
The point of all this? The service posits that the world will really care about knowing the most popular, “hottest” places to be right now. Companies of course will hope their venue is just such a place and can advertise to the hordes.
One new service aggregates a group of services: http://socialgreat.com/
It gives a gestalt view from: Foursquare, Twitter, Brightkite, and graffitiGeo.