It really only assumes the solution gets better if you throw more money at it. And that, frankly, is essentially certain given the fact that they need to pay for human eyeballs. Also, the problems facebook has are somewhat misrepresented often - they don't actually need to moderate everything equally intensively - most of the impact will be there with the things that get shared publicly and are viewed often. Comparing the amount of moderator time with total posting volume is thus misleading, trying to make it appear like a hopeless case. But look at it the other way around, not from the poster's side, but the reader's - clearly reads aren't distributed evently; so it's plausible you might catch 90% of post readungs with just 10% of post writings (I don't know the proportion, but I'd expect it to be heavily skewed).
I'm not familiar enough with how large scale moderation works, so is this solvable, with near unlimited funds?