I disagree with what you say but I will defend to death your right to say it.
This is a contrived example, of course, but it demonstrates a simple idea: (some) words kill.
Facebook is a private company. They have no duty to uphold free speech. However, they do have a duty to do what's in their own self-interest. Being seen as a haven for hate speech is probably not it.
In fact, I really don't see how it's defamation to claim that someone is innocent of genocide.
Imagine a holocaust surviver with a business. Imagine a client that is convinced by a lying holocaust denier. Business owner loses client.
In more general terms, lies cause damage in indirect ways. That you cannot prove such a thing in an individual case does not mean no damage is done. The aggregated effect of holocaust deniers convincing a few more people that the Jews are a lying bunch necessarily causes damage.
Can I imagine him with antennae, too?
You haven't addressed my point, and you're repeating points you've made elsewhere in this thread. You're making the same assumptions (that deniers are malicious "liars", and not simply morons), and informing us only that you're afraid of certain kinds of speech.
Was your point that 'defamation' has a strict meaning in law and that I was wrong in suggesting that these deniers are guilty of defamation in that strict sense? In that case, that is probably true, I'll trust you on that.
My previous comment was intended to explain that I did not mean it in such a strict sense. My argument was not that it is wrong because it is that exact offense that is punishable by law. The fact that we have laws covering parts of that behaviour merely illustrates that we consider it wrong. But that only parts of the behaviour are covered does not mean that the other parts are not considered wrong. Some behaviour is not outlawed because it is not feasible to formulate proper laws to cover the behaviour: even in countries where holocaust denial is forbidden, most cases of holocaust denial cannot possibly be prosecuted.
As an aside, in my book, a 'liar' does not necessarily have malicious intent. He may just be ignorant: morons and liars are not mutually exclusive.
Finally, seeing what demagogues have been capable of: yes, the power of words is scary. Fear is a bad counsellor, but still a warning to be heeded after the fear has passed over you and through you.