This article asks how we should research museum communication with audiences through social media. We argue that museums and audiences co-construct one another on social media, and we explore how particular modes of communication and discursive genres serve to generate mutual online positionings. Based on in-depth analyses over three months’ Facebook communication at nine Danish museums and applying theories and methods from discourse analysis, we offer an analytical model of social media communication; and we examine how processes of co-construction are established, upheld, modified and developed. We find that museums and audiences alike largely co-construct one another along familiar lines of institutional authority and that more dialogic modes of interaction mainly result when museums harness audience knowledge resources. We put our results into perspective in terms of recommendations for museum professionals that offer a sobering empirical corrective to the often celebratory claims made to the de-stabilizing of institutional authority and control wrought by social media.