In addition to formal terms of service and contracts between platform owners, users, and other stakeholders, there can be seen an implicit social contract taking place in online platforms, and influencing the social dynamics, such as trust, expectations, and perceived social justice, taking place within platforms, and driving their growth and success in the background. This paper examines the nature of that social contract, to better understand the complex social dynamics taking place in online platforms. To accomplish that objective, we draw from classic Enlightenment thinkers, e.g., Rousseau, Locke, and Hobbes, to analyze key aspects of social contracts, which we define as the alignment of stakeholder interests, stakeholder support, economic and social justice, and transparency of expectations. As our main contribution, we develop a conceptual framework for the analysis of platforms based on social contract theory, the Platforms as a Social Contract framework. The applicability of the framework is illustrated through a case analysis of YouTube, a popular online content platform. The rich understanding provided by the social contract perspective, embodied in our framework, entails many potential advantages to platform owners, including understanding user motivations and reactions so that effective platform governance with maintaining a sustainable solution to the chicken-and-egg problem becomes possible. While individual platforms may come and go, each faces the same fundamental social dynamics that can be explained and understood by applying the social contract framework presented in this research. This research shows how the framework can be used for analysis of online platforms, as well as suggests future research avenues for developing deeper understanding of platforms as a social contract.