You don't have a constitutional right to post on Facebook. When you invest your life into platforms run by for profit corporations, you agree to play by their rules. Merging state and big tech is not going to help.
Well, that depends on who says you don't. If the government says so, they are wrong, because you do have a constitutional right enforceable against the government to post on Facebook.
The idea of saying "you don't have a constitutional right to post on Facebook" is that you don't have such a right enforceable against Facebook.
Which is true. But under current US law, you do have a civil right enforceable against any public accommodation to be offered the same service that they offer to the public generally.
Which is why OP describes the U.S. enacting legislation creating a statutory right.