The mobilizing potential of the internet has been widely recognized but also sharply criticized. We propose and test in two studies that the social affirmation use of social media motivates individuals for collective action to achieve social change. In Study 1, we surveyed participants of a university occupation and found that enduring participation was predicted by social affirmation use, mediated by group identification. In Study 2 we experimentally tested our hypothesis, the results of which confirmed that the social affirmation use of a forum (and in particular its interactive aspect) motivated individuals for collective action to achieve social change. We discuss the theoretical and practical implications of our findings for theory and research on the mobilizing potential of the internet.