I don't think it's 0%. That would imply that Facebook has no negotiations power in this dispute. But I think quite the opposite is true: Zynga cannot really afford leaving Facebook platform right now, so they don't have any credible threat, other than creating more bad publicity for Facebook.
Based on how things are evolving recently, it doesn't look like Facebook is paying too much attention to it.
Any guesses as to the terms? = ) I'm thinking ~0.00% instead of 30% fees for the credits.
Edit: I meant that I suspect it's much less than 30%, not really 0.00%