It would work for Facebook users, but not for Facebook the company. This is essentially what Reddit does. It mostly works for communities about headphones, bic

It would work for Facebook users, but not for Facebook the company. This is essentially what Reddit does. It mostly works for communities about headphones, bicycles or water bottles, but not Donald Trump, piracy or child porn. Reddit had to close some subreddits down, not because their users complained, their users were perfectly happy and considered the moderation to be just fine, but the harm to society / the company's PR image / advertisers was too great.