The use of Social Network Sites (SNSs) has grown to become a ubiquitous aspect of daily life in most 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. Because of this concern, there is a practical interest within the field of social psychology to better understand why and how individuals engage with online platforms such as Facebook, Instagram, and YouTube. 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 mixed methods study therefore sought to contribute to developing understandings of SNS use and FoMO. This was achieved by assessing the extent of the relationship between SNS use, FoMO, and four other psychosocial variables. Quantitative and qualitative data were collected from a sample of 218 participants, and these data were analysed using mediation analyses and a thematic analysis respectively. The findings of these analyses revealed social capital in particular to be a crucial aspect of frequent SNS engagement, and the correlation between these two variables was found to be mediated by experiences of FoMO.