> You don't have a constitutional right to post on Facebook.

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.