Problematic social media use is becoming a significant social and clinical concern, and there is growing research interest in the psychological factors involved, such as personality predispositions and the fear of missing out (FOMO). The present study investigated both the dark triad (DT, namely, narcissism, Machiavellianism, and psychopathy) and trait emotion intelligence (trait EI) in relation to the problematic use of technology and social media engagement and the mediating role of FOMO. A total of 788 people between 18 and 35 years old (M = 24.22; SD = 3.91; 75% women) were surveyed. The results showed that social media engagement was positively related to problematic social media use and negatively related to trait EI. In addition, problematic social media use was positively related to DT and negatively related to trait EI. Fear of missing out was positively related to social media engagement, problematic social media use, and DT, and negatively related to trait EI. Fear of missing out had a mediating effect on the relationship between personality dimensions, problematic social media use, and social media engagement. The extent to which personality traits may underpin problematic use of social media and the practical implications of the findings are discussed.