How do people who lead communities on online platforms join together in mass collective action to influence platform operators? Going Dark analyzes a protest against the social news platform reddit by moderators of 2,278 communities in July of 2015. These moderators collectively disabled their communities, preventing millions of readers from accessing major parts of reddit and convincing the company to negotiate over their demands. This study reveals social factors—including the work of moderators, relations among moderators, relations with platform operators, factors within communities, and the isolation of a community—that can lead to participation in mass collective action against a platform.