http://www.Last.fmis sort of like social networking based around music. Walls, chats, groups, whatever sort of shit like that as well as "radio stations" of music similar to whatever artist/genre you could imagine, sharing and recommending of music with friends ( as well as automated through the stations ), look up future concerts and events, as well as lots of other assorted of junk. Most delicious of these features of the collection and storage of what music you listen to. Via a light weight app, that also functions as a stand alone music player, it "scrobbles" what you listen to and amasses it into your collection.
Its free, but also has some premium version that I've never looked into as I am completely satisfied with the.. erm.. "free" version.