Facebook does not keep the revenue from user charges. But keeps down the cost of advertising and fees from pay on the application window. The way to further development on the system.
In terms of revenue, Facebook does not store the user's expense, but keep the money coming from advertising and fees from payment on applications that developers come to on the system.
In the income Facebook does not collect money from users, but keep costs down from the advertising and fees from the payment on a wide variety of applications in the system on top of the developer.