Listening to the same thing as your friends right now makes a great demo, increases time on Facebook.com, and is a first step in gathering new data that can be associated with facebook uid's.

Once Facebook has the data for what every person in the world (already close to 1 billion active users) listens to at all times (via FB Connect) then an entire ecosystem will emerge from this data set and the graph API alone.

It might even save last.fm, if they can build something compelling on top of that data.