The important part of the First Amendment, which everyone seems to gloss over, is that it is an explicit restriction on the actions of the gov't. The gov't is not allowed to make restrictions on free speech, religion, assembly, etc, except as already noted above.
This has NOTHING to do with whether Facebook says you can't say this or post a picture of that, or Twitter says we will ban you if you support this other thing, or Blue bans you for posting ASCII pics of tentacles doing awful things here.
Those are corporations and they are allowed to restrict whatever they want on their own platforms. If they feel allowing it will cost them more money than it makes them, then by the almighty rules of capitalism that's what they will do. You can shout "free speech!" at them but it's totally irrelevant. FB can be as hot on censorship as they want it to be. If they drive people away and go broke, there's no law against that. A business is allowed to be self-destructive. The gov't can't refuse to give someone a driver's license because they are gay, but a bakery can refuse to make them a wedding cake. If that bakery loses customers but decides it would rather do that than bake a cake for gays, it can.
The First says the gov't can't tell FB "you're not allowed to post that." It does NOT say it can tell FB "you have to allow people to post that." It is a restriction on powers, not an expansion of them. All of the Bill of Rights, the first 10, are like that. Many of the later ones DO grant powers, such as the ones for Prohibition or income tax, but not those.