Platforms govern users, and the way that platforms govern matters. In this article, I propose that the legitimacy of governance of users by platforms should be evaluated against the values of the rule of law. In particular, I suggest that we should care deeply about the extent to which private governance is consensual, transparent, equally applied and relatively stable, and fairly enforced. These are the core values of good governance, but are alien to the systems of contract law that currently underpin relationships between platforms and their users. Through an analysis of the contractual Terms of Service of 14 major social media platforms, I show how these values can be applied to evaluate governance, and how poorly platforms perform on these criteria. I argue that the values of the rule of law provide a language to name and work through contested concerns about the relationship between platforms and their users. This is an increasingly urgent task. Finding a way to apply these values to articulate a set of desirable restraints on the exercise of power in the digital age is the key challenge and opportunity of the project of digital constitutionalism.