There is only one society, and its members ought to have freedom of speech and assembly amongst themselves. Facebook, just like the Internet, is a platform, and it should allow free speech, even if products that are built on top of that platform themselves don't. It seems to be in the nature of networks to be a monopoly; if Facebook or Internet weren't a monopoly, perhaps competing networks could provide free speech and Facebook and Internet could be fully censored. But as it is, censorship and bans diminish the freedoms of speech & assembly of the users.