iLike is by far my favorite social music service. I love the concert / event recommendations because it's a pain in the ass to actually check artist websites for this information when you love a lot of different artists. Their facebook and myspace applications tie directly into the iTunes plugin which essentially creates a feed of the music I'm listening to (deskside only, it would be awesome if they launched an iphone app).

They win based upon their distributed model (which submits the music I'm listening to at any given moment to the places where I'm already social on the web rather than forcing me to maintain a separate web presence on a new site) and the lack of effort I have to put into feeding my taste in music to their database automatically.