In general I would place restrictions on speech which causes the sort of damage any other crime would cause. If someone loses business because of libel or slander it's as bad as theft. The man who incites a mob to violence should be prosecuted along with the mob he incited. As for a private platform, the owner and the users should be bound by the terms of their agreement, and sites should not be allowed to build off of false advertisements. Whatever rules are agreed to need to be enforced consistently. If someone set's up a site and are clear on what is and isn't allowed, anyone who doesn't want to be bound by the rules needs to stay off the site. If the site owner breaks their own terms or puts out misleading advertisements, they should be held accountable.