When political issues are debated in the public sphere, whose opinions do we hear? In broadcast media, political commentary is dominated by politicians, journalists, and political pundits (Page, 1996; Bennett, 1990). In the UK, as in many Western democracies, these actors tend be older, white upper-class men (Duff, 2008). Consequently, the voices of women, ethnic minorities, young people, and everyday citizens are marginalised (Wayne, 2010; Kassova, 2020). Social media allows these marginalised actors to bypass institutional gatekeepers. However, we do not know whether online political commentary is more demographically diverse. This paper addresses this question by examining the most-retweeted actors on Brexit: an issue which has mobilised a vast swathe of the British public. By analysing the top 105 commentators across 10 salient Brexit discussions, it shows that citizen opinion leaders make up a significant minority of the top retweeted accounts. However, stark gender, ethnic and age disparities remain. These results raise significant doubts over social media’s potential to pluralise political debate. Greater attention should be paid to factors that exclude certain demographic groups from the online public sphere.