Social networking sites (e.g. Facebook), microblogging services (e.g. Twitter), and content-sharing sites (e.g. YouTube and Flickr) have introduced the opportunity for wide-scale, online social participation. Visibility of national and international priorities such as public health, political unrest, disaster relief, and climate change has increased, yet we know little about the benefits - and possible costs - of engaging in social activism via social media. These powerful social issues introduce a need for scientific research into technology mediated social participation. What are the actual, tangible benefits of "greening" Twitter profile pictures in support of the Iranian elections? Does cartooning a Facebook profile picture really raise awareness of child abuse? Are there unintended negative effects through low-risk, low-cost technology-mediated participation? And, is there a difference - in both outcome and engagement level - between different types of online social activism? This SIG will investigate technology mediated social participation through a critical lens, discussing both the potential positive and negative outcomes of such participation. Approaches to designing for increased participation, evaluating effects of participation, and next steps in scientific research directions will be discussed.