Mark Zuckerberg was 100% right. Of course Facebook should not ban Holocaust denial.
Banning a few obnoxious hate terms doesn't interfere with anything but a few hateful people being obnoxious. They don't actually serve any purpose beyond annoying people.
Banning people on the basis of epistemic controversy is entirely different. Banning disagreement about fact is banning discourse. And discourse is literally the only way to actually defeat malevolent or erroneous ideas.
If people believe something that is simply wrong, especially something that is truly, factually incorrect, fact is the only way to fix that. Banning Holocaust denial doesn't just mean banning Nazis, it means banning people who heard a couple Nazi ideas and don't have sufficient understanding to identify the ideas as bad. You're not just banning Holocaust deniers, you are cutting off your own ability to explain the error of Holocaust denial to people who don't already understand it by engaging the Holocaust deniers themselves.
And that's exactly how we got in the position we are today with the alt right. Dumb kids finding ideas forbidden to them elsewhere, in obscure venues where there's no one who knows better to take the ideas apart.
Some of us on the left are going to have to get something through our heads – it's necessary to hear uncomfortable ideas, sometimes. It's necessary to deal with those ideas. It's necessary to refute those ideas, rather than hiding our heads in the sand or going to the dean to cry about hate speech.
You cannot defeat ideas by silencing them. You defeat bad ideas with good ideas. Learn how. Stop trying to take the easy way out.