Digital media are often blamed for accelerating the decline of political parties as channels for citizen participation. By contrast, we show that political engagement on social media may revitalize party activities because these platforms are means for both party members and ordinary citizens to discuss politics and engage with and around political parties. Using online surveys conducted in Germany, Italy, and the United Kingdom, we find that party members engage in a wider variety of party-related activities than average respondents, but the same can also be said of nonparty members who informally discuss politics on social media. Moreover, the strength of the relationship between party membership and engagement decreases as the intensity of political discussion on social media increases. This suggests that political discussions on social media can narrow the divide in party-related engagement between members and nonmembers, and to some extent flatten rather than reinforce existing political hierarchies. Finally, we find that the correlation between party membership and engagement is stronger in Germany, where party organizations are more robust, than in Italy and the United Kingdom, highlighting the role of party organizational legacies in the digital age.