This article examines how the UK political organization Momentum uses social media within its campaigning. Drawing on a mixed-method research design, combining interviews with activists in Portsmouth and discourse analysis of content posted on Facebook and Twitter, this article tests whether the leadership provides meaningful influence for members. At the national level, there is little evidence of Momentum fulfilling its "people-powered" vision. Instead, supporters are instructed to undertake tasks at the direction of the leadership. However, this is not a straightforward case of controlled interactivity. The local group in Portsmouth is semiautonomous, providing member-driven advocacy that is coordinated through a Facebook Group. By using social media to underpin different organizational norms and campaigning tactics at different spatial levels, Momentum represents a "movement faction".