Banning speech can and typically has been harmful, especially when governments get involved with doing it. No one is ignorant about this fact. But we're collectively discovering that not doing anything about what can be published and said is just about as harmful, if not more so in concentrated circumstances. There will be irrational people who act irrational on fundamental erroneous information. It's a practical matter, not an ideological one. White supremacist ideas aren't up for debate. There isn't a point in debating them. And the people adhering to these ideals aren't looking for a debate and most definitely won't consider other ideas. You can't debate rationally with someone who hasn't arrived at their position in a rational manner.
There's also something else at play here. We're seeing new systems that replaced old ones re-implement "lost" functionality they thought wasn't needed. Turns out it was, in fact, needed. The old media empire of yesteryear was always subject to regulations around the content they spread.
If bitcoin is an exercise in why we need financial regulations, facebook and other social media networks are an exercise in why we need content and publishing regulations.