This article analyzes social movements' appropriations of social media for challenging mainstream media systems. The study includes two recent Latin American student movements: the Mexican movement #YoSoy132 and the Chilean student movement. A quantitative-qualitative methodology was used to compare their appropriations of social media, which included a statistical analysis of the Facebook profiles of both movements during a selected period of time and Fairclough's critical discourse analysis of the contents posted and interviews with participants. The findings indicate that there are some relevant specific trends in the appropriation of social media for this specific purpose, mediated by dimensions such as the demands, goals, political communication context, online or offline nature of the mobilization, and organizational characteristics of the movement.