Facebook has a major issue issue with their Facebook Credit System that can affect you if your account is ever hacked.
This method will be used to pay for any RP purchases you make, and you do not get to verify anything before it is used. It will not even direct you to PayPal to verify the payment -- it is automatically authorized.
To delete these payment methods:
1) Go to Facebook Account, Account Settings
2) Click the "Payments" tab
3) Click the "manage" out beside "Payment methods"
If you used a credit card, there will be credit card information listed. If you used PayPal, at the bottom it indicates the email address used for your PayPal account.
YOU MUST DELETE THIS!!!! If someone hacks your Facebook account, they can now make Facebook-related purchases automatically without any additional approval or passwords needed! I assume you will need to delete this information after every Zynga purchase.