Little is known about why and how people use multiple social media platforms for political participation, or about the contexts through which social media is appropriated. This paper reports on a qualitative interview study of social media use by politically interested citizens. We interviewed 27 residents of the state of Hawaii who integrated one or more social media tools into their daily lives to participate in the online public sphere. Different social media environments offer both different affordances for action and different audiences, and we describe how media choice is driven by the match between motivations and affordances, and also by the imagined audience. We identified a number of motivations including understanding different viewpoints, formulating perspectives, engaging in positive discourse, repairing Hawaii's image, increasing political awareness and improving civic engagement. We discuss how these goals relate to both intrinsic and extrinsic motivations. Finally, we examine how social media choice and satisfaction were tied to the physical world context and people's sense of the audience within any particular medium.