The question of whether FB as it exists now would abuse such a system (yes, of course they would, they're awful) is separate from the question of whether or not

The question of whether FB as it exists now would abuse such a system (yes, of course they would, they're awful) is separate from the question of whether or not said system would make business sense for a gigantic social network. If FB were the kind of company that could figure out how to make money without ads or taxing 3rd party devs, maybe they'd also be the kind of company that would treat its users as customers rather than products. Hypotheticals all around.