Despite the increasing number of studies on digital technologies, the actual scope of the Internet for creating political discussion and building political support still remains in some areas unexplored by academic research. This paper studies online political participation in the context of the 2012 Mexican presidential campaigns. By analysing the presence of two of the main Mexican parties - Partido Revolucionario Institucional (PRI) and Partido Autonomo Nacional (PAN) – on Facebook, the most popular social website in Mexico, the study shows the importance of pre-existing networks in supporting parties and candidates and the risks of using Facebook Open Groups and Public Profiles for networking. Discourse analysis and network analysis were conducted to show the importance of building up a sense of personal belonging to generate public debates. In line with this idea, special attention is paid to the construction of ephemeral “communities” by recalling shared values and using the presence of hostile outsiders to reinforce the own identity. On the one hand, a network analysis of the Facebook users visiting the parties’ pages suggests the importance of existing networks in fostering online participation and the issues created by the use of public groups. On the other hand, a network analysis of interlinked websites demonstrates the instrumental role played by other websites enhancing the debates by supplying content for the discussion and attracting new participants. Finally, the paper presents the unpredictable and sometimes undesired results of using open groups which are created in the need of specific websites to be acknowledged by Facebook’s visitors.