The lesson of the Facebook experience is that this space is too important to be left to the market. As a society, we need to decide what kind of competition we want to allow and what kind of competition we want to discourage. Perhaps we want sharp limits on how much time children are permitted to spend on these sites. Perhaps we want any site with more than 100 million users to have to open its algorithm to public review. Perhaps we want any site with more than a billion users to take editorial responsibility for speech on its platform.