Unless I'm mistaken, Facebook is not a government and ignorant, uncivilized people getting whats coming to them features heavily in his works. While he certainly frowns on censorship, people intolerant and incapable of behaving themselves are usually thrown out an airlock or otherwise completely ejected from society to roam the surface Mars alone. If you're going to argue the Heinlein route, perhaps Holocaust Deniers need some frontier justice instead?

The issue is not that people can't listen to holocaust denialism or general nazi apologia. Issue is they do and then take action based on that and other people then suffer consequences. Including violent action.

Also, given that Heinlein fiction can be also quoted literally in defense of military dictatorship, it is not like random quote of him was any argument on this topic.

Or any other old big name. The whole "I cut good sounding quote of someone famous or of context" thing is not an argument.