Mobilization is central to the emergence, survival and success of armed groups challenging the state, and has lately expanded to new arenas with the rise of social media. Using a new dataset of rebel group Twitter use, we examined the topics contained in rebel group social media communications to understand how different messaging strategies impact civilian engagement with rebel messages. Rather than benefiting solely from direct calls to action, we found that rebel groups also increased civilian engagement through indirect messages of self-promotion. While direct appeals received more engagement than indirect appeals, their effects were tempered by audience fatigue when relied on too heavily. We additionally found that including images further enhanced the impact of a mobilizing message. These findings expanded our understanding of rebel communications and mobilization, with important implications for combating the use of social media as a recruitment tool for violent extremism.