Online social networks are increasingly important information and communication tools for young people and for the environmental movement. Networks may provide the motivation for young adults to increase environmental behaviors by increasing their knowledge of environmental issues and of the specific actions they can take to reduce greenhouse gas emissions. This study examined an application within Facebook.com--the largest online social network in the world--that allowed users to post climate change news stories from other websites and comment on those stories. A survey revealed that users of the social networking application reported above average knowledge of climate change science and that self-reported environmental behaviors increased during young people's involvement with the "Facebook" application. Focus groups indicated that peer role modeling through interaction on the site motivated pro-environmental behaviors, that is, behavior that seeks to minimize the negative impact of one's actions on the natural and built world. Participation in a community of like-minded users spurred many participants to learn more about climate change and do more to limit its impact. (Contains 4 tables.)