This paper develops an endogenous model of institutional and professional domain change. Traditional accounts of domain change focus attention on how professional expertise is extended to new areas of practice. This form of domain extension is typically both deliberate and contested. However, domain change can also occur in a somewhat quotidian and uncontested fashion when professional expertise is extended intra-organizationally. We analyze the ways in which the domain of accounting expertise is reconstituted in new social media – Facebook, LinkedIn and Twitter – in Big 4 accounting firms. Using content analysis and interview data we show how social media professionals, in pursuing their own professional project, generate change in the professional domain of accountancy. Our analysis demonstrates that the institutional work of domain change occurs through three related activities: boundary work, rhetorical work and the construction of the embedded actor.