As liberal democracies grapple with the evolution of online political extremism, in addition to governments, social media and internet infrastructure companies have found themselves making more and more decisions about who gets to use their platforms, and what people say online. This raises the question that this paper explores, who should regulate extremist content online? In doing so the first part of the paper examines the evolution of the increasing role that social media and internet infrastructure companies have come to play in the regulating extremist content online, and the ethical challenges this presents. The second part of the paper explores three ethical challenges: i) the moral legitimacy of private actors, ii) the concentration of power in the hands of a few actors and iii) the lack of separation of powers in the content regulation process by private actors.