Along with the increasing number of companies introducing Enterprise Social Networks (ESN) in recent years, research on ESN user behaviour has proliferated. Yet, a detailed analysis of factors driving ESN user behaviour, that is, how users participate in ESN, is missing. Addressing this gap, in this paper, we explore ESN user behaviour and factors influencing usage in an Australian professional services firm. Based on a case study including 14 interviews with regular users of the case company’s ESN, we identify and characterise six general dimensions of ESN user behaviour. In addition, our analysis indicates ESN user behaviour to be influenced by a mix of individual factors and organisational factors. The contributions of this paper include a conceptualisation of user behaviour as well as a set of factors shaping ESN usage. For the management of ESN communities, our findings are hoped to inform initiatives aiming at reinforcing user engagement over time.