Time for the Facebook class action to sting them hard.
There absolutely is precedent that a private company or private property can be considered a public forum and therefore has to respect free speech / assembly rights.
The Colorado Supreme Court ruled in Bock v Westminster Mall that because they allowed a range of activities at the mall and also allowed government offices (recruiters for the armed forces) to set up kiosks that they had become a latter day public forum and could not deny free speech / assembly rights. This case in particular is pertinent.
The US Supreme Court ruled in Marsh V Alabama that owners of a private company town could not prohibit the distribution of religious tracts because they had treated the town as a public area.
Basically if a private business does certain things that act like a public forum then they become a public forum.
Facebook, Twitter, and YouTube allows business to use them, private citizens to use them, cities, etc. They also allow the government to use them. There are also a wide range of activities. Community postings, fund raising, etc. additionally, unless a person keeps their content private, the content is visible to anyone without an account, meaning it is public.
They have become a de facto town square and must not prohibit free speech activities. There is grounds for class action against all of the big 3.