This book argues that social media are redefining the ways in which citizens engage with political information and that, overall, these changes have positive implications for political participation in Western democracies. By allowing users to encounter clearly identifiable political viewpoints, facilitating accidental exposure to political news, and enabling political actors and ordinary citizens to reach voters with electoral messages designed to mobilize them, social media make a positive contribution to users’ repertoires of political participation. Moreover, political interactions occurring on the web 2.0 do not only benefit citizens who are already involved, but boost participation across the board. This is because social media offer both additional participatory incentives to the already engaged and new political opportunities for the less engaged. The overall outcomes of these developments are an increase in aggregate levels of participation and an expansion of political equality across Western democracies. These patterns are also shaped by systemic characteristics and institutions, whose hitherto neglected role we uncover based on innovative theorizing and unique cross-country empirical data.