Over the past two decades, a number of digital platforms have been developed with the aim of engaging citizens in scientific research projects. The success of these platforms depends in no small part on their ability to attract and retain participants, turning diffuse crowds of users into active and productive communities. This article investigates how the collectives of online citizen science are formed and governed, and identifies two ideal-types of government, either based on self-interest or on universal norms of science. Based on an ethnography of three citizen science platforms and a series of interviews with their managers, we show how different technologies – rhetorical, of the self, social, and ontological – can be diversely combined to configure these collectives. We suggest that the shift from individual projects to platforms is a defining moment for online citizen science, during which the technologies that sustain the collectives are standardized and automatized in ways that make the crowd appear to be a natural community.