It doesn't necessarily mean that. 2/7ths of the world's population logging in to FB once every two months would result in the same MAU number, even with zero people logging on _every_ month.

Moreover, these MAU numbers probably include people like me, who receive a push notification of a message sent to a group, click it once to view the message, and quit the app after 2 seconds.

It's still a large number, though!