In this study, we explore social media use by politicians by investigating affordances, effects and associated constraints of introducing social media within eParticipation. Based on an interpretive case study in the Indonesian context, involving members of both national and provincial parliament, this study unearths internal and external affordances offered by social media. Internally, among politicians, social media enable, for example, idea storming and information sharing; while externally in the relation with their constituents, social media is used, for instance, for opinion gathering and political networking. These affordances yield both internal and external effects, such as broadened perspective, improved internal political supports, better public control, and better information for decision-making. This study also unveils constraints that hinder politicians in utilizing possibilities offered by social media in the eParticipation context. Theoretical contributions and practical implication are also presented, including classification of internal and external social media affordance, identification of constraints and affordance effects. This study is adding contribution to practice by helping politicians to design social media strategy for eParticipation.