Today, Internet users start off using heavily SNS(Social Network Site) such like, Facebook, Twitter. The reason of the growth of using SNS would be closely related to the various services of gaming, playing, entertainment items, sharing information etc., provided by the SNS, technically, the most important one out of the services provided would be behaving of sharing knowledge among people who connected and networked in the site. In sum, we assume that the users may communicate well each other and pay attention to build closely a social network using that kind of activities. However, nevertheless the new trends of communications and sharing knowledge become popular, researchers have just began the research issues in explaining why Internet user rush into SNS and enjoy the time in there. Therefore, we investigate on the reasons of posting knowledge voluntarily in the SNS and how others response to the posted information and actually affected by the behavior. We appled personal relation and social identity theory for this study, which personal relation in SNS may affect on social identity and make them produce knowledge generation. We found that social identity and involvement in SNS is closely related and influence knowledge creation and generation. This empirical study resulted in the importance of social relations in SNS, which leads to a sharing knowledge.