We study the dual roles of the stability of an individual's social network and social contagion on individual behavior in the mobile Internet setting. We use a panel dataset containing all mobile records for a sample of 3G mobile subscribers. Our data includes information about their frequency of mobile Internet usage, and their communication patterns across voice calls and messages, which allow us to map any dynamics in their social network. We find three main results. First, users with high network stability have a low intrinsic tendency to engage in content usage and generation on the mobile Internet. Second, the extent of positive social contagion effect is mitigated for users with stable networks. Third, we find that network stability is a significant predictor for individual behavior even after controlling for network closure. We discuss the implications of these findings for social network theory, social contagion and managerial practice.