Drawing on evidence from the 2011 Egyptian uprising, this paper demonstrates the effect of two social media platforms – Facebook and Twitter – on a discrete facet of revolutionary mobilization: successful “first mover” mobilization. Specifically, it argues that these two platforms enabled the revolution’s first movers to organize and execute a large, nationwide, and seemingly leaderless first day of protest on January 25, 2011, which signaled to hesitant but sympathetic Egyptians that a revolution might be in the making and convinced them to join subsequent protests. Using qualitative and quantitative evidence, including surveys, interviews, and scraped Twitter meta-data, it first demonstrates that those who used social media during the revolution tended disproportionately to be among those who participated on the first day of protest. It then discusses three mechanisms that linked these platforms to successful first mover mobilization: 1) movement recruitment, 2) protest planning and coordination, and 3) live updating about protest logistics. The paper not only contributes to debates about the role of the Internet in the Arab Spring and other recent waves of mobilization, but also demonstrates how scholarship on the Internet in politics might move toward making more discrete, empirically grounded causal claims.