Since both ‘conflict and co-operation are at the heart of employment relations’, unions need to strike a balance between mobilizing workers against employers and ‘social dialogue’ when communicating with members and the public. Drawing on a case study of unions and grassroots communication on Facebook during sector collective bargaining, this article develops a framework of politico-communicative logics that inform the social media strategies of unions and their grassroots: a logic of mobilization and a logic of settlement. These logics are reflected in how unions and grassroots engage with followers, their choice of words and the topics they address. These theoretical developments on politico-communicative logics on Facebook open up the door for further studies of intra-organizational union communication in an increasingly digitalized world.