Do social media incite protest movements, how? How does onsite protest shape online communication dynamics on social media? We examine the oftdebated relationship between social media activity and protest, considering the Yellow Vest or Gilets Jaunes movement in France. We use data of temporal and spatial distribution of Facebook activity and onsite events over a year of social mobilization to address these questions within a system of interacting online and offline activity. This study contrasts with others by analyzing mutual influences between online and offline domains while considering collective information diffusion in social networks. The results show that the relationship between social media activity and onsite protests is not constant nor one-sided, but mutual, and that it changes over the course of the collective action in specific and measurable ways. Using online and offline data we show different mechanisms by which both domains of activity influence each other. Specifically, the creation of Facebook groups impacts the outbreak of protests, and in turn, this outbreak modifies the parameters of information diffusion between users on Facebook. In addition, Facebook content that pertains to the frames of the movement and mobilization affects onsite mobilization efforts. Our work contributes to the modeling of social movements as systems of online and offline events, expanding on studies that traditionally examine the impact of online activity over onsite protest. Keywords Social network analysis • complex systems • human dynamics • social media • political participation • collective action • protests • social movements • Facebook • Yellow Vests • France.