Social media platforms are often credited with empowering grassroots movements in the pursuit of political freedoms. In this paper, we show how social media can also be exploited by political elites to undermine democratic institutions, using the January 6th, 2021 Capitol insurrection as a case study. We present three main findings. First, by exploiting plausibly exogenous variation in Twitter usage, we document that social media exposure predicts participation in the Capitol attack, donations for anti-democratic causes, beliefs in election fraud, and support for the January 6th rioters. Second, Donald Trump‘s tweets questioning the election‘s integrity were followed by spikes in "Stop the Steal" activity on Twitter and pro-Trump donations originating from high Twitter usage counties. Third, the insurrection and Trump‘s account deletion were followed by a decrease in the public expression of toxic political and "Stop the Steal" messaging by pro-Trump users on Twitter, but had little effect on privately held beliefs about the election outcome and pro-Trump donations.