Facebook does not keep the revenue from user charges. But keeps down the cost of advertising and fees paid on application and the length of the follow-on system developers.
In terms of revenue, Facebook does not store the user's expense, but keep the money coming from advertising and fees from payment on application. That way the 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 application on the sanctions were the developers, and system on top of.