There's no absolute requirement for using Facebook to communicate with others over the Internet, so there's no sensible reason why they (or anyone else) should be compelled to host content they don't like.
Just because certain platforms are popular doesn't really change things from a legal or a moral perspective, and it might be because of their editorial discretion that they are popular in the first place.
Just because certain platforms are popular doesn't really change things from a legal or a moral perspective, and it might be because of their editorial discretion that they are popular in the first place.