A body of research suggests that social media has afforded new opportunities for orchestrating mobilization in autocracies. However, the mechanisms linking online coordination with offline mobilization are rarely demonstrated. We address this lacuna by exploring the impact of Farsi-language social media posts that called for protest on precise days and locations in Iran during the 2017 ‘Dey Protests’. To conduct our analysis, we match a dataset of posts with an original protest event catalogue. Our results show that if a district was the subject of a protest call, it was much more likely to witness higher levels of mobilization on the target date. This relationship was especially pronounced for calls that received more online engagement. The findings suggest that the digital commons can play a role akin to an analogue protest flyer: social media posts can inform broad audiences of the where and when of upcoming mobilization.