Social network sites (SNSs) have attracted millions of users who interact with each other and with companies. However, few studies have examined the impact of knowledge sharing through electronic word of mouth (eWOM) in the context of SNSs. This paper investigates the relationship among the use of SNSs, users’ social capital, knowledge sharing, and eWOM. The results show that the intensity of use of SNSs is positively related to trust and identification which have a positive effect on eWOM quality. In addition, eWOM quality has a positive effect on knowledge sharing. Female users feel more strongly about eWOM quality when they trust others, or when they perceive that they belong to their SNS community when they use their SNS. Furthermore, female users feel more strongly about knowledge sharing when they perceive that eWOM quality is good. This study provides the theoretical framework of the relationship between eWOM and knowledge sharing on SNSs from the perspective of social capital. Practitioners could use this study as a rationale to utilize SNSs internally for organizational use, and externally for marketing purposes.