I have been using Spotify for over a year now and I believe it has redefined streaming music.
Now I think it's redefining personal and social music. Their latest iteration of their desktop application integrates beautifully with Facebook. You can see a friend's profile, which includes their favorites artists and saved playlists.
If you subscribe to a playlist, it links back to Spotify on Facebook.
There is a new "Library" tab in the left hand navigation now.
If I can store all of my music here, stream new music and find cool artists, and interact with my friends on Facebook, I don't think I'll ever use iTunes again. And Songbird doesn't have a chance unless they really start following Spotify's lead.
Send me an email if you are interested in Spotify or tweet at me, and I will see if I can get you an invite.