Most studies on social network evolvement focus on the effect of structure aspects (e.g. social network size, degree, centrality etc.) of social network sites (SNSs), but fail to pay attention to SNS users’ behavior as a potentially influential factor (e.g. usage pattern, content flow). Many online social network sites have faded into oblivion not because of the lack of user base, but due to the inactivity of users online. This research adopts the case study approach to investigate why users eventually abandon SNSs. Users adopt SNSs mainly to gain social capital. However, as users gain more social capital, they may eventually experience a “backfire”, which we term the “social capital paradox”. Adopting the lens of expectation disconfirmation theory (EDT), we attempt to elucidate the mechanisms underlying the eventual abandonment of SNS