Rhapsody Facebook Plugin. I'm new to Facebook but, the plugin is pretty cool. You can search for music (all the normal artist/album/song) play music right in the site, and even send recommendations to your friends.
Dropping the player on somebody's wall while being simultaneously unobtrusive is pretty nice too.
Since my facebook account is pretty new, I just treat the site like a lightweight music player.
If you're into the whole social networking thing, give it a try.