Maartje Boer

INTRODUCTION 13 1 users typically present themselves in an overly flattering way (Fardouly & Vartanian, 2016). Intensive exposure to such idealized portrayals of others may elicit upward social comparisons, such as the perception that others are more popular, attractive, or successful in life (Pera, 2018; Verduyn et al., 2020). Alternatively, high SMU intensity may go at the expense of meaningful offline activities, such as face-to-face socializing with friends or homework (Underwood & Ehrenreich, 2017). In addition, adolescents who spend more time online may be at higher risk for cybervictimization (Sampasa-Kanyinga & Hamilton, 2015). In turn, these factors may harm adolescents’ mental health. In line with these concerns, large-scale cross-sectional research among U.S. and U.K. adolescents showed that higher intensity of SMU is associated with less happiness and more depressive symptoms (Kelly et al., 2018; Twenge, Martin, et al., 2018). Also, longitudinal studies showed that higher levels of SMU intensitypredictedsubsequent depressive symptoms andoverall internalizing problems (Frison & Eggermont, 2017; Riehm et al., 2019). Furthermore, meta- analytic findings suggest that a higher intensity of Facebook use is associated with more depression, anxiety, and other mental health problems (Frost & Rickwood, 2017). In addition, several cross-sectional studies on adolescents showed that problematic SMU was related to worse psychological health, including higher levels of depressive symptoms, anxiety, and stress (Bányai et al., 2017; Pontes, 2017). Moreover, meta-analytic findings using adolescent and young adult samples showed a positive relation between problematic Facebook use and psychological distress (Marino et al., 2018b). Together, these studies indicate that both the intensity of SMU and problematic SMU are related to lower mental health. However, many questions about these associations remain unanswered, which we outline below and aim to answer in this dissertation. Are SMU Intensity and SMU Problems Independently Related to Poorer Wellbeing? A major disadvantage of the studies discussed above, that is central to this dissertation, is that these either studied the intensity of SMU or problematic SMU in relation with wellbeing, instead of simultaneously. However, the two different SMU behaviors are correlated with each other and the abovementioned studies suggest that both the intensity of SMU and