Yeah, not in today's world where they are sock puppets for the government.

It doesn't matter because no one else can just infringe on your rights either. Rights are not about just protecting you from government, they're there to protect you from other people.

Except if you have a de facto monopoly on social media which is the digital equivalent of a public forum then you have the ability to effectively curtail free speech.

It is an infringement of free speech as a concept.

It is not an infringement of US law as the relevant protections are limited in scope to governmental actions.

Obviously US law and even more so the supreme court's interpretations of them are flawed, both on a moral level (big corps should also not be allowed to censor speech) and a logical level (censoring speech is free speech, corps are entitled to human rights).