> Why would and should facebook be treated differently than other websites? Having a large amount users and money doesn't suddenly make them a public entity.

Because FB is not targeted at specific communities of people like HN, and less so reddit, it's welcome to everyone. Everyone is in fact actively encouraged to join. That's their mission. Private spaces that are open to the public often have first amendment protections:

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=139661