I've been using Foursquare for a few months now and it actually is kinda fun. It's basically the "Check in" part of Facebook but "on steroids" so to speak. You earn badges for different things, you earn points for checking in (the points don't really seem to get you much of anything other than gloating points though from what I've been able to tell so far), you can earn "Mayorship" of different check-in places, which again, being the Mayor of a place earns you nothing but bragging rights, and a few other things. Maybe the best part about it is if you check-in to certain places, for example, an AppleBee's restaurant, you might get surprised with some kind of special deal there, like a discount, free meal, free side dish, or whatever.

The way that I see it,  if I'm going to be checking in anyway (on Facebook) then I might as well do it through Foursquare, since you can set Foursquare up to automatically post the check-in to your Facebook page as well (and your Twitter page if you choose). The only disadvantage of using Foursquare to check-in to Facebook for ya is that you can't manually choose your Facebook friends to tag as being at the check-in place with you, like you normally can on Facebook. If you want that you could just do like I do and use Foursquare for most of your check-in's, and then use Facebook to check-in only the places you want to tag Facebook friends with. Although, since you also have a friend's list on Foursquare, if those friends are also added to it and have foursquare it'll automatically say that you're there with them as well (more or less).