Granted, Marsh has not been very relevant in First Amendment jurisprudence of late, but it is standing precedent, and it could very well be extended to cyberspace.

Also, private actors are not exempt from legislation restricting the use of censorship. There is a First Amendment right to not be made to speak, but if you're a common carrier you're considered to not be speaking when carrying others' speech; social media could well be classified as common carriers or some similar (new) classification with similar effect. Given this it is a fair question as to what the appropriate public policy is as to censorship on private platforms. You might say that no public policy is appropriate on that question other than "hands off", but then again, since Congress has the relevant power to legislate, others could reasonably propose limits on censorship by privately-owned social media.

"Start your own" is good advice, but "lobby Congress" and "file suit" are also available avenues, and for some those are more realistic than the first.

I suspect that before long we will find out if Marsh applies to social media. I'd love to read legal analyses of that matter.