Judges. The question is mainly whether there should be some rules independent of those of the companies that the content must follow, and people who feel treated badly need to get their rights against those rules in a civil lawsuit, or whether more should be allowed first until there is a civil or penal lawsuit that might stop it. (It is already a mixture of both so it's a matter of degree.)

I personally prefer an emphasis on the first solution because it's better to combat the widespread lack of civility in social media, which I believe to harm society substantially, but I also understand the people who prefer the second model.