Pré-publication, Document de travail: How does social media affect the composition of political protests in the United States? Using early adoption of Twitter at the 2007 South by Southwest (SXSW) festival as a plausibly exogenous source of variation in county-level Twitter penetration (Müller & Schwarz, 2023), and comprehensive data on protest events, this paper finds that Twitter penetration increases the frequency of protests overall, but also radicalizes them. Twitter disproportionately fuels protests with participation of "extreme" groups-groups that are particularly militant, radical, or hateful. These effects do not depend on the topic of the protest nor political leaning. I also present survey evidence suggesting that coordination is not the only mechanism driving these results: An increase in county-level Twitter penetration implies an increase in respondent's willingness to justify violence against other people, normalizing the participation in extreme groups and extreme protests.