In the last decade, the platform economy has transformed urban life, using crowdsourcing technology to change the way people eat (e.g., Instacart, UberEats, Doordash), move (Lyft, Uber), work (TaskRabbit), and travel (Airbnb). An accompanying, but understudied, change is this economy’s dramatic reorganization of urban civic participation, as platforms mobilize their users to fight the market and labor regulations supported by local civic groups and market competitors. My dissertation uses a mixed-methods approach to examine the social bases of short-term rental markets (e.g., Airbnb) and their consequences for urban civic organizing and policymaking. I argue that the very way these markets are socially organized—relying on civiccapacities, labor and housing inequalities, and technologies that blend “sharing” with transactional forms of exchange—shape the local politics over their regulation, creating new allegiances between civic groups and the corporation.