The use of Social Network Sites (SNSs) has grown to become a ubiquitous aspect of daily life in developed countries throughout the world. This rise of social media has resulted in increased public concern regarding the way in which individuals engage with SNSs, and the consequences of frequent SNS use. The Fear of Missing Out (FoMO) is an example of a social psychological phenomenon which has recently received attention as a significant factor associated with experiences of SNS engagement. The following study sought to contribute to developing understandings of SNS use and FoMO. This was achieved by assessing the extent to which FoMO mediated the relationships between SNS engagement and four other psychosocial factors (psychological need satisfaction, social capital, public self-consciousness, and public self-monitoring). Quantitative data were collected from a student volunteer sample of 218 New Zealand SNS users and analysed using a series of regression analyses. Relationships between each of the assessed psychosocial variables and rates of SNS engagement were found to be mediated by FoMO. The findings of this study support the view that FoMO plays an important role in understanding the range of complex and interrelated psychosocial factors relating to rates of SNS engagement.