What does your Facebook app do? Does it have a strong viral component? Those almost never are able to direct much traffic back to your site, but they can often do what your site does, possibly better, and spread faster.
A user can send a song from Cherrypeel to any friend (on their wall or in a private message) and they can vote on the song directly in facebook. They don't need to sign up for the app to listen to the song but will need to sign up tosend the app on to another friend. http://apps.facebook.com/cherrypeel/
So is your app meant to be standalone at all? Or just to interface with the site?
This app feels like something I'd just skip by, though maybe the experience would be much better if I were sent a song. But just getting there directly, there isn't much to do. There's one song to play, which is cool, but how do I find more?