Facebook had a net income of $3.6bn from 1.86bn monthly active users, or $1.93 per user.
I don't think many users are going to work for what Facebook could reasonably pay out of that....
Worse, any remotely attractive payment might well lead to the rapid creation of billions of fake accounts.
[1] http://static2.uk.businessinsider.com/image/589259d8dd0895fb...