Audio Scrobbler is a website (and suite of plugins for various audio players) that keeps track of what music you listen to and makes suggestions based on that music. There are some social networking aspects, as you can keep a list of friends and neighbors. When you’ve listened to enough music, there are all kinds of interesting statistics to dig into.
While I’m on the subject of social networking sites, here’s a lazy web idea: What if there was some site and/or software to keep track of all your social networking accounts in one place? You could make a change to a single profile, and it would propagate to all your accounts all over the place. (Or better yet, allow you to choose which accounts to update selectively.) I’d use it.