With the advancement of social network services (SNS) such as Facebook and Twitter and information technology, diverse communication and the expansion of personal networks have been increasing explosively. As SNS users have grown rapidly, research on their usage behaviors has become critically necessary, and the need to examine this through empirical studies has been raised. This study aimed to examine the mediating role of the desire for online identity expression—the desire to express oneself online—from a theoretical perspective regarding the motivations and outcomes of SNS use. To this end, an empirical analysis was conducted using SmartPLS 2.0 with 232 respondents who use SNS. The analysis results showed that individual innovativeness and social norms had a positive (+) effect on the desire for online identity expression. Through this, it was found that the desire for online identity expression also had a positive effect on the enjoyment of knowledge sharing and SNS involvement. However, the desire for online identity expression did not play a significant role in network formation and maintenance. This suggests that merely expressing one's online identity is insufficient for forming and maintaining networks. Nevertheless, although the desire for online identity expression did not directly affect network formation and maintenance, it was found to have a significant indirect effect through SNS involvement. Therefore, summarizing the above findings, it was confirmed that when online identity expression leads to active participation in SNS, this can positively influence network formation and maintenance as well as the enjoyment of knowledge sharing, thereby enhancing SNS usage and outcomes.