As part of the ongoing trend of layering digital data on top of the real world, Facebook has entered the location-based services game. Until now, this space has been occupied by Foursquare, Gowalla and a few others.
Existing location-based services have tended to focus on developing a game-like user model based on personal status. Checking in can get you special status: such as "Mayor" for a particular location when using Foursquare, or Gowalla's digital souvenirs.
This social-status-based gaming aspect has probably been an important and effective driver of initial user growth for these services -- who doesn't want to be "mayor" of their favorite hang-out spot? And when you don't have a lot of social connections already inside the service you're