Enterprise social networking (ESN) is a relatively new phenomenon and not yet fully understood. An important reason for this lies in the fact that ESN are built on malleable technologies that do not lend themselves to immediate forms of workplace usage determined or prescribed by their features. Rather, their potential only manifests when people make sense of and incorporate them into their day-to-day work practices. In this paper we consolidate three years of case study research into ESN adoption and use. We are able to demonstrate empirically the contextual nature of ESN as an emergent phenomenon that takes on different forms in different organisational contexts. Drawing on this data we derive a theory of ESN usage, which employs a set of use cases to provide a better picture of the possibilities of ESN when appropriated into team, project or large enterprise contexts. This allows reflecting on ESN as a contextual phenomenon and social infrastructure and builds a basis for further research.