I've increasingly felt that fb, twitter etc with their filters and algorithms are more publisher than platform.

They indisputably go to great lengths to control what you see ergo they are not simply facilitators.

They should be liable for what is published on their platform but they certainly cannot be guilty of _not publishing_ something.

By refusing to publish something, they are no longer acting as a neutral platform.

Losing the immunity that they get by being a neutral platform, means that current content on FB, that they were previously not liable for, they would now become liable for.

For example, if a user posts something illegal on FB, then Facebook could be sued, even if they remove the content as soon as they became aware of it. (In the same way that the NYT is responsible for the content on it's writers).

Think about how phone companies have immunity from someone making illegal phone calls, and then phone companies becoming liable for damages that they previously were not liable for previously.

I'm not a lawyer, what are the legal basis for that "immunity" and the fact they lost it somehow?

For telecomms, this is known as “common carrier” status; it means you are not liable for what goes through you, but to keep that status you must not filter anything either.

If you do filter, you’re supposed to lose that status - and then you become liable for what goes through your pipes.

I’m not sure there’s a similar legal concept for publishers.