Foursquare is fun. It's on online location based game that is currently available in a select few cities in the United States. When you go to places you check in on Foursquare you earn points for your check ins. And when you play the game you earn badges such as Bender, Local, and Super User. For all I know there may be a Walk of Shame, but I have not yet seen it. You also get to become "The Mayor" if you've got more check ins than anyone else at a particular place. It's kinda fun to compete and oust folks. As a matter of fact it's a lot of fun. More than fun, it is addictive. I love it.
Fred Wilson quite correctly points out that the value of a social network is derived with frequent usage and a large friend network. Foursquare has captured the former through their addictive game mechanism. The large network however, is an issue.
I have approximately 1,900 followers on Twitter, 700 connections on LinkedIn, and 300 friends on Facebook. On Foursquare I have 31 friends. And 9 requests from others to become their friends that I doubt I am going to accept. These numbers are partially based on the relative size of the networks and my use of them. But there is something else at play. Think about these networks. The size of these networks are inversely related to their intimacy. The less intimate the larger the network.
Foursquare is very intimate. Very. It tells people where I am. When I am there. This leads to much more discriminate friend acceptance. Anybody can follow me on Twitter. On Foursquare I only friend people whom I would welcome having a spur of the moment in person conversation. That number is small.
And while I love Foursquare, unless they can overcome the intimacy effect or achieve extreme density, they are going to be small as a result.