If I understand correctly, Facebook is the worst advertising opportunity on the planet. Nonetheless, Fortune 500 companies who are crazy to use that have pushed its valuation to the highest that has ever been for an internet IPO.
Having said all that, Facebook bills once your account reaches $30. It is not even possible to prepay if you request it.
This billing practice is even more annoying than Facebook's failure to create an ad system that is profitable for everybody.