Many critics think social media poses a novel threat to liberal democracy. Seeking to divide Americans, agents of the Russian government bought ads on Facebook. Extreme speech also finds a home on the internet, fostering conflicts that appear to generate more heat than light. Governments and consumers worry about “fake news” designed to misinform readers for fun, profit, and power. And yet social media has made more information more widely available at less cost than any technology since the printing press. Less reliant on gatekeepers than traditional media, the new purveyors of news arguably better satisfy the diverse preferences of the American electorate. Following up on the Project on Political Reform at the Harris School of Public Policy at the University of Chicago, we will consider the troubles and triumphs of the social media platforms that promise to host American political debate for generations.