I'll make a principled stab at it.
If you ban Holocaust deniers, then you're in the business of drawing lines, proclaiming what is truth. This is a powerful position and many will wish to use it for more than just the Holocaust. They will initially do so by presenting a variety of outrageous content which maybe should be banned, applying public pressure, and their initial targets will be reasonable.
But the outrage industry is big these days, and its targets are many. I shall present some recent evidence of its working. Consider a figure like, say, Bruce Gilley, who I was just reading about this past week: he's an academic who says non-mean things about colonialism. A public outrage drive, started by Maoist philosopher Joshua Moufawad-Paul (author of "The Communist Necessity") got his publisher to cancel a biography of Sir Alan Burns; similar outrage drives have previously attempted to see his doctorate revoked. I have not read his scholarship and have no reason to suppose his work to be excellent; however, I should prefer that academic degrees are not revoked after the fact for failing to conform to any particular orthodoxy.
Yet these are the times that we live in: we are increasingly censored by the mob. Facebook content moderation is an obvious target and there is no doubt will be targeted. The categories will expand. Facebook's resistance to such measures will either be heroic, or profoundly inadequate.
So consider: Is the Republican party under Donald Trump a hate group? Is it legitimate for them to have Facebook operations? Surely if you work in Silicon Valley you know someone who considers it a hate group, and believes it should not be permitted a Facebook presence. That is where this ends up. It is not a long and tortuous path. And as hateful as Holocaust denial is, it is possible that Facebook might make the world a better place in the end by allowing it, that they may say, "We are not the content police. No one is."