Banning hate communities does work though [0]. I assume that the results are similar for twitter and facebook. "Hateful" Reddit or FB communities also don't allow "free speech". The moderators will ban people who go against the grain. There is no free exchange of ideas or disputing as if you're going against the grain you'll just get banned from that community (but not the platform). As such, either the platforms allow the hateful communities to just exist in their 'safe space silo' or they de-platform them.
[0] http://comp.social.gatech.edu/papers/cscw18-chand-hate.pdf