And if you don't think Zynga is at risk from a change of policy by Facebook you're kidding yourself.
It's a gamble. A huge gamble. Zynga may in fact be large enough now to survive war with Facebook but they'd be savaged and greatly diminished by it (IMHO).
Look no further than the move to Facebook Credits [1] to see who really wields the power in this relationship. Facebook has exacted a 30% cut, forced on app developers (interestingly I haven't seen quite the same outrage at this as I have about iOS subscription changes).
[1]: http://www.allfacebook.com/zynga-facebook-credit-2010-09