This paper seeks to specify and empirically test a two-step theoretical model of how internet use is affecting individual's likelihood of engaging in politics, specifically in terms of their likelihood of voting. Building on existing theories of participation and mobilization we argue that the key to understand the social media's impact on individual political engagement lies in its power to promote informal political discussion which increases levels of interest and efficacy in politics. We test our hypotheses using structural equation modelling techniques and a unique data source: a face-to-face survey conducted after the UK 2010 General Election. We use our results to reflect on the appropriateness of existing theories of participation for understanding online participation and the extent to which the new media can be seen as providing a channel for mobilization of previously less engaged individuals.