This study investigates, over an 18-month period surrounding the 2016 Brexit referendum, the commenting activity of nearly 2 million Facebook users engaging with political news from British media or with the posts of referendum campaigns. We ask whether citizens’ engagement with political news on Facebook motivates their participation with political campaign posts, and we examine whether users commenting on campaign pages trend towards ideologically reinforcing media. Overall, we find comparatively low levels of commenting activity on the official referendum campaigns vis-à-vis the media, and the majority of users (70%) commented only once. Looking at the subset of users commenting on both page types (“cross-posters”), we identify a general spillover effect from media to campaign pages, suggesting a positive correlation between political interest and online participation on Facebook. Reverse spillover occurs immediately around and after the vote, with Remain cross-posters active on the Guardian while Leave cross-posters’ media engagement registers as more diffuse.