This paper examines the mechanism by which social media use impacts a citizen's online political participation behavior. Using the lens of Downs' Theory of political participation, we propose a model of political participation based on cost-benefit approach. Based on previous studies, we argue that Social Media use increases one's social capital and reduces one's cost of participation and thus increases one's benefits of participating in online political activities. The results obtained confirmed our hypothesis about the impact of social media use on citizen's social capital and in-turn their online political participation. Our results add to the existing literature on political participation and social media use by modeling the relationship between the two. The outcomes of study also have important implication for public organizations as it can help them to promote participation among people using social media. Implications for theory and practice are discussed.