INTRODUCTION: Smartphones are ubiquitous in the digital society. Although this powerful device is useful because it supports and simplifies many tasks in everyday life, a growing number of researchers is concerned that smartphones might have detrimental effects on the human brain and related psychological processes. Evidence indicates that social media platforms such as WhatsApp and Facebook are essential drivers of smartphone usage. Thus, a critical, yet unexplored issue is how excessive use of those platforms is related to Smartphone Use Disorder (SUD). Furthermore, since the roles of life satisfaction and fear of missing out (FoMo) have been demonstrated to be of particular importance for Internet Use and Internet Communication Disorder, those constructs were examined in the context of SUD. METHODS: In total, n = 2299 participants filled in questionnaires assessing SUD, WhatsApp and Facebook Use Disorder, FoMO, life satisfaction, and reported owning a WhatsApp and Facebook account. RESULTS: The study revealed that SUD was linked to WhatsApp Use Disorder, and to a lesser extent, to Facebook Use Disorder. Associations between SUD and WhatsApp Use Disorder were most strongly pronounced in females. Additionally, FoMo predicted SUD, WhatsApp, Facebook Use Disorder, and (fully) mediated the relationship between life satisfaction and those variables. CONCLUSION: The findings of the present study suggest that WhatsApp use plays an important role to understand SUD. Regarding the mediating role of FoMO, the present work might help disentangle inconsistent results on the link between life satisfaction and (excessive) social networking sites use.