I don't know how far Facebook or Twitter have to go to prove violation of their user agreements. Those things are really vague and to date they've been really hesitant to enforce them. I suppose you could attempt to push back on an equal protection basis? But not the First Amendment. I still don't see how that applies. And I think the really vague user agreement probably covers them pretty well here.
Can social media have any say in what is posted there? What do you think the legal basis for that is? If they get some say, then we're talking about a matter of degree. And being inconsistent could be a really bad business decision but probably isn't against the law.
I mean, the mods here can ban ULM or ULL users if they want. They don't until someone forces them to, but they could. Should they not be able to do that?
I think if Facebook limited themselves too far, they'd lose enough money to regret it. Maybe they've done that now.
An additional problem with the way we talk about all of this is that we're lumping a lot of bad arguments in with whatever good arguments there might be (and even some of those are not really relevant to this specific thing people are mad about).
Far be it from me to defend Big Tech. But I'm not really sure how they meet the normal standard for being a monopoly or how they'd even be able to violate First Amendment rights.