This article analyzes the citizen movement in defense of the public health system and against the proposal of merging two hospitals in the city of Granada. The merger was scarcely covered by the mainstream media until a physician denounced the hospital system reform in a Facebook video that soon went viral. From that moment, a social online mobilization began facilitating greater involvement of citizens and civic organizations, reaching traditional political actors. Using a multimethod approach and based on content analysis, interviews, and social networks analysis, this case illustrates the mobilizing capacity of social media, highlighting the ability to overcome the traditional lack of agency that is associated with virtual mobilizations. It also demonstrates the ability of social media to set the social agenda and to influence the mainstream media, which are then “forced” to introduce the demand into their agenda. Finally, it also shows how new online social leaders become microcelebrities with high mobilizing capacities.