Social media may help civil society organize and mobilize for different campaigns. However, the extent to which social media campaigns simply recruit like-minded individuals as compared to exerting a causal impact on joiners' attitudes is difficult to disentangle. We test both the organizational and transformative potential of a civil society campaign in a randomized field experiment deployed via Facebook or an email newsletter in collaboration with a Bulgarian environmental campaign. As expected, we find that Bulgarian Facebook users who are active in pro-environmental groups, and those who decide to follow the campaign, are more highly educated than those who decide to stay at the sidelines. Moreover, beliefs in the effectiveness of civic society, character traits, and prior activism systematically predict whether a Bulgarian Facebook user decides to join the cause on Facebook, or subscribe to the email newsletter. In contrast, we find little evidence that the campaign affected opinions, knowledge, or self-reported behavior. We conclude that social media campaigns that are commonplace among civil society organisations are effective at selecting activist-types, but changing the views and behaviors of the broader social media population may be more difficult than assumed.