I have the opposite opinion: these companies do collectively cartelize public speech. People who get banned from one platform are frequently banned from others soon after, especially if notorious. These companies also all have content policies prohibiting the same constitutionally-protected speech, e.g. on some subjects relating to the recent election.
Censorship doesn't have to be binary to be censorship: an environment in which your reach is reduced to practically nothing if you have one viewpoint and amplified into the stratosphere where you have the opposite viewpoint is an environment antithetical to the purpose of the first amendment, which is about creating an open and honest marketplace for ideas. Being able to talk about some idea in your kitchen but not on the internet is an inadequate level of free speech protection for today's world.
There's nothing wrong with requiring neutrality from social media companies. Common carrier regulations wouldn't violate their free speech rights: these corporations aren't speaking in their own voices, but are instead carrying the speech of others. And besides: corporations are not natural persons and we should abridge their rights in order to preserve the rights of real people.