Aye, cost per action. Generally people use email submit / zip submit offers.
For example, take a look at one of those pet applications - they'll have that you can do x, y, and z to get your pet grow so much daily. Some people want their pet to grow faster, so they'll be an offer fill in this form to get x points extra or similar.
If you get what I meant, that's a base model for making money with Facebook apps
