I believe that they use Facebook because it's easy to use, not because it's the only available option outside the dark web. That is to say if Facebook removed them, they'd switch to some other platform.

Might slow them down (won't have FB's built-in audience), but not nearly as much as only being available on the dark web. Hate campaigns worked pre-internet, and they'll still work post-FB. Maybe not as efficient, but I don't think efficiency is the limiting problem there.