Due to the rapid development of social media and mobile technologies, more and more users access social media via mobile devices. Existing research focuses on user satisfaction with either social networking services or mobile applications. Perceived benefits, such as perceived usefulness and perceived enjoyment, are important antecedents of user satisfaction with mobile applications in general. Mobile social apps are different from general mobile apps in the sense that the intensive social connections and influence among users may affect user satisfaction with the product. Very few studies have examined the factors influencing user satisfaction with mobile social apps. In this study, we built an integral research model on user satisfaction with mobile social apps by drawing on the theories of network externalities and herd behavior. By conducting a survey with the users of a popular mobile social app, WeChat, we empirically show that network externalities and herd behavior have significant influence over users’ perceived benefits toward the mobile social app. We also find the significant mediating effects of perceived benefits on the relationship between network externalities and user satisfaction and the relationship between herd behavior and user satisfaction. Our findings provide useful insights to mobile social app developers and marketers as well as mobile social app users.