Online social network sites are often used for personal purposes during working hours. Whether or not such use interferes with and impairs work performance is obviously of organizational and economical interest. This impairment hypothesis was empirically tested by the current study. A total of 11,018 (5,656 male) employees participated in a web-based cross-sectional survey distributed in the online edition of several Norwegian newspapers. To investigate the relationship between the use of online social network sites for personal purposes during working hours and self-reported work performance, these measures were included with additional questions about demography (age, sex, education, relationship status, professional position) and personality (Mini-IPIP; Extroversion, Neuroticism, Agreeableness, Conscientiousness, Intellect/Imagination). The data was analyzed by hierarchical regression analyses, where absolute (usual performance) and relative (usual performance compared to one's judgment of the usual performance of most workers in similar a job) self-reported work performance comprised the dependent variables. Demographic variables were entered in block one, personality variables in block two, and use of online social network sites for personal purposes during work hours was entered in the third and fi nal block. This variable was signifi cantly and negatively related to both absolute and relative self-reported work performance, and explained 1.1% and 1.2% of the variance in these variables, respectively. The fi ndings suggest that the use of online social network sites for personal purposes during working hours has a negative eff ect on self-reported work performance, although the effects were very slight. The results might have been infl uenced by self-report bias.