Rdio's already on Facebook's hours-old Open Graph. The Rdio app lists the songs you're listening to in your News Feed or Ticker, and you can click any Rdio tracks that appear on Facebook to listen to them immediately.
The app automatically connects the service to your Facebook account without downloading anything. It also automatically creates an account for any of your friends who have never used Rdio whenever they click a Ticker or News Feed link to listen along. [Rdio]