This is true, but remember that there's a lot of terrible things that the Constitution doesn't prohibit, e.g., murder. The fact that something isn't Constitutionally prohibited doesn't mean that there shouldn't be a law against it.
I'm not saying Facebook is infringing my rights (I explicitly meant to say the opposite). But I do feel strongly that they are guilty of censorship, in the literal sense.
They might be in the legal right, sure. But you're never in the moral right if you're guilty of censorship.
Using your same logic, should Starbucks be required to let you stand in their store and say racist things? What if the manager thought you said something racist and kicked you out when you didn't actually say anything racist, is Starbucks guilty of censorship?
I don't agree. The snarky and smug "nuh uh, cuz it wasn't the government" responses [often maliciously] assume that complaints about censorship are talking about the first amendment, but I think people are generally aware that censorship can come from corporations too and their concerns are not limited to apparent violations of the first amendment specifically but are concerned with the more general principle of free speech (which predates the first amendment.)
> Censorship, the suppression of words, images, or ideas that are "offensive," happens whenever some people succeed in imposing their personal political or moral values on others. Censorship can be carried out by the government as well as private pressure groups. Censorship by the government is unconstitutional.[1]
Facebook has enough power over communication at large to effectively suppress speech should they choose. Censorship does not have to be perpetrated by governments. Hence the terms self-censorship and corporate-censorship[2].
What people really mean when they talk about censorship is first amendment rights. Here, you are correct. The first amendment protects against government censorship only - nothing Facebook does could infringe your first amendment rights.
[1] https://www.aclu.org/other/what-censorship
[2] https://en.wikipedia.org/wiki/Corporate_censorship