"Widespread concern about the effects of social media on democracy has led to an explosion in research over the last five years. This research comes from disparate corners of academia: departments of political science, psychology, law, communication, economics, and computer science, alongside new initiatives in data science and even artificial intelligence. A new field is forming, and it is time to take stock of what we know, what we need to know, and how we might find it out. That is the purpose of this book"--