This chapter explores the materialities of social media as archives of social movements’ user-generated content. By conceptualizing social media as archives, we unpack how the materiality of social media shapes production and storage of mediated communication by social movements resulting in processes of over- and under-representation. These processes are not controlled by activists, but largely by social media logics. The loss of control over their own data leads to social movements’ practices of avoidance, which might result in invisibilities and incomplete archives. We conclude by arguing that inclusive archiving projects of social movements’ mediated communication and writing a history from below in social media should rather be non-commercial projects in close collaboration with activists. Writing a history from below by creating archives where activists lay claim to their own representation and regain control over their data is understood as a way to make visible imbalances of power.