"The Holocaust happened. You can deny it all you want to, it happened". Calling for a mob to kill Jews is not. Facebook users may not advocate violence or plan criminal activities, but merely expressing incorrect opinions is permissible. So were the countless violent pogroms that preceded the Holocaust, when hateful people spreading lies and libels saw to it that Jews were marginalized, dehumanized and murdered.
The comments by Zuckerberg drew fierce criticism and appeared to undermine Facebook's latest effort to root out hate speech, violence and misinformation on its platform.
But beyond those guidelines, there are large gray areas. That's fake when they say those children were actors. Or mocking someone's premature death - something that is also prohibited? Quite a predicament, Facebook and Zuckerberg find themselves in.
So, where it can, Facebook stays out of policing content. "It is utterly abusive and Mark Zuckerberg must take responsibility for excising it from Facebook". If the posts are false, Facebook will "significantly reduce the distribution of that content" in the News Feed, he said.
The episode was an unwelcome distraction for Facebook after it held a briefing on the company's new policy to remove bogus posts likely to spark violence.
YouTube recently started doing this too.
However, it is now not clear how Facebook will determine whether content could lead to violence. "It's costly and hard", said Steve Jones a professor of communications at the University of IL at Chicago.
Ms Lyons said she did not have any more details to publicly share about the policy at this stage. They were all founded by engineers as tech companies that shun labels such as "media" and "editor".
Then there's the legal shield.
The company has struggled to balance its belief in free speech with those concerns, particularly in countries where access to the internet is relatively new and there are limited mainstream news sources to counter social media rumours.
Zuckerberg added: "These issues are very challenging but I believe that often the best way to fight offensive bad speech is with good speech". But Trump campaign veterans say that, privately, Facebook employees were full of praise for how the campaign was able to leverage the Facebook platform.
Facebook CEO Mark Zuckerberg gestures while delivering the keynote address at the f8 Facebook Developer Conference 2014.
In a statement to CNBC, a Facebook spokesperson said that, "There are certain forms of misinformation that have contributed to physical harm, and we are making a policy change which will enable us to take that type of content down". "Later we also identified that they had set up a fake account and fake pages under the banner of, connected to this thing, DCLeaks, in order to seed stolen information that they had gotten to journalists", Zuckerberg said.