Free speech doesn't protect my right to blare Nazi propaganda 24/7 into your living room using focused sound technology. Nor should it.
In the US, it doesn't even grant a person the "right" to blare Nazi propaganda 24/7 using a licensed FM radio station.
Many people seem to take as granted that free speech is a virtue that an entity like Facebook should support, but their statements assume without evidence that free speech protection is a virtue in that context.
This is a common argument but it is simply not true. Someone cannot tape your mouth shut because they aren't the government. The government (at least the US government) is saddled with the responsibility to protect your rights, even from other entities. I cannot search your house just because I'm not a cop. What good is a government if it only protects your rights from being infringed by itself?
With regard to the particular topic being discussed, that is, a private entity moderating speech on its own property, https://en.m.wikipedia.org/wiki/Marsh_v._Alabama is very directly applicable, and more specific to the situation itself. The only sticking point to be hashed out might be "terms of service" which might constitute a private agreement and this precedent may not directly apply. Of course we shall see, because I'm willing to bet these cases will be brought to court within the next 5 years.
People who would otherwise post on Facebook about Holocaust denial can go out on the streets and talk about it to their friends or others. But, they can expect a punch in the face if they do. I think that is what is happening here. People are spewing nonsense, and a non-governmental entity (Facebook) is punching them in the face (not letting them say it on their platform).
Like the grandparent post said, this is intolerant. But it's following the paradox to allow more tolerance in society.