This paper argues that social media companies’ power to regulate communication in the public sphere illustrates a novel type of domination. The idea is that, since social media companies can partially dictate the terms of citizens’ political participation in the public sphere, they can arbitrarily interfere with the choices individuals make qua citizens. I contend that social media companies dominate citizens in two different ways. First, I focus on the cases in which social media companies exercise direct control over political speech. They exercise quasi-public power over citizens because their regulation of speech on social media platforms implies the capacity to arbitrarily interfere with citizens’ democratic contestation in the political system. Second, companies’ algorithmic governance entails the capacity to interfere with citizens’ choices about what mode of discursive engagement they endorse in their relationships with fellow citizens. By raising the cost of deliberative engagement, companies narrow citizens’ choice menu.