Audoioscrobbler.com

"Sick of Friendster? Bored of MySpace? There's a new social networking site in town, and it's got a twist that will keep even the most jaded of Internet users interested--at least for a time.
Audioscrobbler.com is an online social network that rolls the concepts of Friendster and iTunes into one. Like Friendster or MySpace.com, users create profiles and make networks of friends by linking themselves to others. The fun part comes with no more effort than that. Audioscrobbler uses a plug-in to a computer's music player to track what songs a user is listening to. So at the end of the day, you can check to see what your friends have listened to most, ranked by most recently played, most-played artists and most-played tracks."

[CNET]