People use social networking sites because their friends are on it. There’s no value in being an early adopter of an empty networking tool. So the essential problem of launching a social networking site is how to build mass until you reach critical. (Metcalfe’s Law: the value of a network is the number of users squared.)
It’s only so difficult to get people to log on to your site when they’re sitting around at home on a Sunday night. It’s much more difficult to get people to log on when they’re out in the world doing stuff, so the mass-building problem is much worse for geosocial networking.
Foursquare’s solution is to turn geosocial networking into a roleplaying game: your online character gains power-ups when you check in. The most common power-up is that when you check in to a location more than anyone else you become the “mayor”. In the system this doesn’t give you anything more than bragging rights, but FourSquare has huge potential for linking with the real world:

The obvious business model is to sell site-specific power-ups. For example, checking in to Joshua Tree in Murray Hill, New York gives you a douchebag power-up (make no mistake: this is great publicity). Another example is that the Hotel on Rivington in New York gives you the “I’m on a boat” power-up.
Don’t dismiss people who would buy a $16 cocktail just to ride a dolphin doing flips and shit. Foursquare is the anti-World of Warcraft: it makes a game out of going to new and interesting places; your friends can suggest places for you to go. And guess what, it just launched in Vancouver.