The negative impacts of social media have given rise to philosophical questions around whether social media companies have the authority to regulate user-generated content on their platforms. The most popular justification for that authority is to appeal to private ownership rights. Social media companies own their platforms, and their ownership comes with various rights that ground their authority to moderate user-generated content on their platforms. However, we argue that ownership rights can be limited when their exercise results in significant harms (...) to others or the perpetration of injustices. We outline some of the substantive harms that social media platforms inflict through their practices of content moderation and some of the procedural injustices that arise through their arbitrary application of community guidelines. This provides a normative basis for calls to better regulate user-generated content on social media platforms. We conclude by considering some of the political and legal implications of our argument. (shrink)