I would argue that last.fm wasa first massively social music service, and it also shown that social side is not all bright. I made a few good friends there but the amount of hate and loathing in shoutboxes astonishes.
Another thing is that social network tracks social graph, not "friends" in the narrow sense; some of people in my social graph are likely to listen to the same kind of music than I (active friends); some are likely to listen to white noise (extended family, former close friends, collegues and people I studied with). You basically need circles!