The first amendment constrains the government. A social media site censoring your post does not infringe on your first amendment rights. It limits your speech, but the 1A protects your right to be free of government interference with your speech.
It only specifically constrains Congress. It does not mention state governments.
In addition, this falls under freedom of association; people (individually and in groups) are free to choose who they associate with, and include or exclude anyone they like based on any criteria they like (with certain specific exceptions regarding protected classes). "We reserve the right to refuse service to anyone" - for instance, a restaurant or store (or an individual at home) can kick out someone being disruptive, and so can a web host or social media platform.