I've been thinking about this recently.. If your facebook app does well, then you will pay for it in terms of servers and bandwidth, but where can you make returns?
Unless you can somehow draw traffic to your site through your facebook app I think it might be an uphill battle. I heard reports that iLike actually LOST website traffic after the release of their facebook app.
You are getting some degree of brand recognition but the user base really isnt yours if they never use your product outside the facebook sandbox.
Any thoughts?