This has come up in cases of corporation-owned/built towns meant to house workers that then violated their residents (i.e, employees and their families) free speech rights in said towns. Constitutionally protected speech was upheld vs Private Property rights.
There have been very good arguments made that the same should be applied to Facebook, et. al.
Obligatory XKCD: https://xkcd.com/1357/
Facebook is a de facto monopoly in the social networks space, and it's deciding to deny service to specific individuals based on their political opinions. It's not something I'm willing to shrug off as unimportant.
And I have to disagree with xkcd too on this one. Protecting the freedom of speech means setting limits to the freedom of others to harm you when they don't like what you say. People are free to disagree and switch channel/ unfollow/ stop buying a newspaper/ etc. But harassment and mobbing are a different matter.