For over two decades, political communication research has hailed the potentially reinvigorating effect of social media on democracy. Social media was expected to provide new opportunities for people to learn about politics and public affairs, and to participate politically. Building on two systematic literature reviews on social media, and its effects on political participation and knowledge (2000–2020), and introducing empirical evidence drawing on four original US survey data that expands for over a decade (2009–2020), this Element contends that social media has only partially fulfilled this tenet, producing a Social Media Democracy Mirage. That is, social media have led to a socio-political paradox in which people are more participatory than ever, yet not necessarily more informed.