Cooking the books is far worse than pushing misleading metrics to customers, at least as far as corporate ethics goes. The risks would be enormous, and the benefit would be temporarily propping up the stock price, which is not all that helpful if you aren't currently raising money. Facebook most likely isn't defrauding investors simply because it goes against Facebook's interests to do so.

Of course, companies come up with their own metrics all the time, but they have to be published alongside GAAP (Generally Accepted Accounting Principles) metrics.