The problem is that the rev/user isn't constant. If they get more users, they earn more from each user. And conversely, if they lose users, they earn less from

The problem is that the rev/user isn't constant. If they get more users, they earn more from each user. And conversely, if they lose users, they earn less from everyone. So even if a small percentage of their users opted to pay monthly, that would have a negative impact on facebook's earnings on the remaining population.