User content vs targeted content
I think the salient point here is that if a web site acts only as a neutral conduit for user content, between users, then it's fair to say that they are acting as a "common carrier" and aren't responsible for that content.
On the other hand, if they start "promoting" some content over other content, using opaque alogorithms that the user has no control over, and inserting advertising based on that content, then they should be seen as a publisher, and are responsible for the lawfulness of that content.
I think FB have gone quite a long way over the line from one (where they used to be) to the other (where they clearly are now). The relevant lines are where they started "curating" content, and where they started targeting advertising based on a profile built up from your "likes" and "views" and yor demographic data. There's no way they should be able to claim anything other than responsibility for those adverts, especially if they are political adverts with no fact-checking, during an election.