Legally, of course, FB is completely right. Morally - try to replace "white nationalism" with, say, "communism" (one can argue communists killed a lot of people, so why not?), "Christianity", "Islam", "Black separatism", "Wiccans"... I could continue forever, but I hope you get the point. If you're consistently fine with any $GROUP put there, no matter how much you love it/disgusted by it, then your position is consistent and you are one of about 0.001% of people that has a consistent free speech position. For the rest, the answer would be "but this is completely different - $INGROUP can not be banned, they are a nice people that sometimes are misunderstood, but $OUTGROUP are definitely all violent bigots that should be banned everywhere for everybody's safety".

Of course, FB does not owe anybody to neither have consistent free speech position, nor support free speech at all, nor be an inclusive platform. They could convert their site to the fan club of Jar Jar Binks tomorrow and make every user to pledge loyalty to Supreme Lord Jar Jar or take a hike. Completely within their rights. Presenting themselves as an open platform while executing a clear political censorship is less morally clear (though of course still legally completely OK). It doesn't matter that political censorship is now directed to the group that is outgroup for most people here - it always starts that way. It never ends there.