Drawing on evidence from the 2011 Egyptian uprising, we demonstrate how the use of two social media platforms–Facebook and Twitter–contributed to a discrete mobilizational outcome: the staging of a successful first protest in a revolutionary cascade, or, what we call “first mover mobilization.” Specifically, we argue that these two platforms facilitated the staging of a large, nationwide, and seemingly leaderless protest on January 25, 2011, which signaled to hesitant but sympathetic Egyptians that a revolution might be in the making. Using qualitative and quantitative evidence, including interviews, social media data, and surveys, we analyze three mechanisms that linked these platforms to the success of the January 25 protest: 1) protester 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.