A model of interactive information technology (IT) usage that integrates network externalities with traditional usage motivations is proposed and is validated by a survey of instant messaging (IM) usage by university students in Taiwan. Network benefit, found to be a significant usage motivation, arises from direct and indirect sources, conceptualized as referent network size and perceived complementarity, respectively. Network benefit has a direct effect on user intention to use interactive IT and an indirect effect mediated by perceived enjoyment, and in turn it is affected by perceived complementarity. IT vendors can enhance product value by investing in value-added complementary products and services. Implications for IT usage theories and managerial practice are discussed.