Maybe some people leap in without thinking. I'm on it but I have only been building up the repertoire of things I do and features I use very slowly. It's too complicated and unpredictable but some of it works okay.

"Holocaust denial" is tricky because the laws about it vary so much between nation states, from positively encouraging it to using it as a pretext for censorship of statements which have nothing to do with the Holocaust. You'd need a comparative law degree just to work out the differences between what was permitted in England and Scotland. And if FB took it on themselves to impose restrictions that ignored the legal systems of countries they operate in, they'd immediately run foul of generic freedom of speech laws.

There isn't a simple answer, but they could (a) put more money into dealing with this sort of thing and (b) be more open about what their enforced policies actually are.