Social media technologies are making fast inroads into organisations. In the context of knowledge-intensive work the propositions of improved communication, information sharing and user involvement seem particularly promising. We study the phenomenon of Enterprise Social Networking (ESN) in the context of Professional Ser-vice Firms (PSF). Our case study investigates emerging knowledge work practices on the ESN platform Yammer within Deloitte Australia. We perform a genre analysis of communication data and uncover a set of emerging practices. We reflect on our results in the context of the knowledge-intensive nature of professional service work. We find that Yammer in the case company has become 1) an information-sharing channel, 2) a space for crowdsourcing ideas, 3) a place for finding expertise and solving problems and 4) a conversation medium for context and relationship building. We conclude by positioning ESN in the well-known 3-C model for classifying collaborative ICT.