Facilitated by the widespread adoption of smartphones, applications (apps) on smartphones such as WeChat and WhatsApp have seen rapid and explosive growth. These apps are generally referred to as second-generation mobile instant messaging (SMIM) services. Unlike first-generation mobile instant messaging (FMIM) services (e.g. Short Message Service), SMIM services typically support multimedia contents and are embedded within social networks, which may have a bearing on the post-adoption behaviour of users in particular. However, prior studies on the post-adoption usage of SMIM services have a limited understanding of the effects of social network. Network tie strength, as a configuration of social network, has an important impact on users in SMIM services. In order to explore the effects of social network on users’ continued usage intention in SMIM services, we propose and empirically test an integrated model by identifying the antecedents such as tie strength, satisfaction, and perceived critical mass. This study contributes to existing IS post-adoption literature by understanding and capturing the role of social network (i.e. tie strength) in SMIM services. Implications for theory and practice are discussed.