Revenue in Q4 2014 was $3.58 billion, monthly active users was 1.393 billion which then yields a revenue of $0.86 per active user and month. With $10 per user and month they would earn a lot more and at the same time with a lot less effort.
I doubt that - most users would probably not pay $10 per month to remain on facebook. The more people who left then the more incentive there would be for more people to leave, since part of the barrier to exit is that many of a user's friends and contacts are using facebook and don't have a presence on alternative social networks.
