I think the pressure to keep it family friendly, and not be an anything-goes shitshow, is more from advertisers who don't want their ads to appear beside anything objectionable. I don't known how that works with FaceBook or Twitter (who are bleeding advertisers), but with print media advertisers apparently get to specify lists of words (pertaining to objectionable subject matter) that disqualify articles from appearing alongside their ad.

The majority of FaceBook's business is advertising, so that is presumably their primary concern (or at least was, until Trump's threats have become an issue).