Zynga has revealed that as part of their agreement to use Facebook credits they receive advertising revenue from Facebook. A recent S-1 filing indicates that the the publisher made $22.8 million in advertising revenue, a small part of its over $575 million in total revenue.
Each month during the term, for all Properties on which you implemented, during the previous month, the Facebook Ad Unit, we will pay you a percentage of net revenue ( Ad Share ) arising from such properties for the previous month, says the filing reads. You acknowledge and agree that Facebook Credits will be the sole and exclusive payment method that is used, accepted or otherwise made available on or in connection with all covered Zynga Services during the term.
Source: VentureBeat
