Retaining users and facilitating their continuance usage are essential for mobile service providers to increase both revenues and profits. This research integrates the perspectives of network externality and switching costs to propose a model for investigating the continuance usage of mobile instant messaging services. Network externality arises from both direct and indirect sources and is conceptualised as referent network size and perceived complementarity, respectively. The results show that perceived complementarity is the main factor affecting perceived enjoyment, whereas referent network size has a strong effect on perceived usefulness. Switching costs is the main factor determining switching barriers. Perceived usefulness, perceived enjoyment and switching barriers determine continuance usage.