In this article, we explore the potential role of social media in helping movements expand and/or strengthen themselves internally, processes we refer to as scaling up. Drawing on a case study of Black Lives Matter (BLM) that includes both analysis of public social media accounts and interviews with BLM groups, we highlight possibilities created by social media for building connections, mobilizing participants and tangible resources, coalition building, and amplifying alternative narratives. We also discuss challenges and risks associated with using social media as a platform for scaling up. Our analysis suggests that while benefits of social media use outweigh its risks, careful management of online media platforms is necessary to mitigate concrete, physical risks that social media can create for activists.