The scary part for Facebook isn't them leaving, it's them reducing the Facebook component to a viral spread mechanism for another, more gaming-focused social network. This will allow them to shift the page views and revenue away from Facebook.

There is already a game doing this largely through FB connect and I believe it's over 1m monthlies. I cannot remember the name right now but it was Evony-ish. It spreads the same way as any app, and even has an app, but all the app does is get you to play outside of FB.

Facebook's ad CPMs are comical, but given the number of pageviews they get and the marginal cost (has to be very near 0) of serving them losing that will hurt a lot in the short run.