In this paper, we explore strategies that institutional entrepreneurs use to legitimate entreprise social media in a context where ESM interest vary across and within organisational units. Using an in-depth case study of a financial firm we contribute to IS-legitimation research in several ways. First, we outline an institutional logics template that future IS-legitimation research can use to conceptualise organisational and technological values, in order to effectively assess legitimacy gaps. Second, we address calls by Flynn and Du(2011) for studies of IS legitimation in pluralistic contexts by proposing that when ESM interest vary across and within organisational units legitimation seekers may successfully gain and sustain participation by applying pragmatic selection, conformation and advertising legitimation strategies,.Finally, we advance a professional ESM design that can be used to gain participation for ESM before network effects are generated. This responds to calls for ESM designs that can overcome the network effects paradox and contributes to our overall motivation to conceptualise strategies that can be used to gain participation for ESM in mature settings.