Aim: The aim of the paper was to investigate the relationship between the level of the sense of meaning life and mental wellbeing as well as the fear of missing out in relation to gender, social class and family affluence. Method: The study used data from a pilot study conducted in Poland in 2017 before the main HBSC 2018 survey (Health Behaviour in School-aged Children). An anonymous survey was conducted in lower-secondary schools, in accordance with the international HBSC research protocol. The analyses were performed in a group of 539 students (46% of girls) attending grades 1–3 of lowersecondary schools (mean age 14.75 ± 0.99 years). The following tools were used in this research: Fear of Missing Out Scale (FOMO), Purpose in Life Test (PiL), Well-Being Index (WHO-5), and Family Affluence Scale (FAS). The results of univariate non-parametric analyses concerning the mean level of the above indices were presented. A generalised linear model for FOMO as a dependent variable was used with stepwise introduction of variables as a part of multivariate analysis. Results: Students with a high level of PiL showed a much higher life satisfaction and a lower level of FOMO than those with a low level of PiL. PiL increased in more affluent families. A significant correlation between PiL and FOMO was found in both genders, while a significant correlation between PiL and WHO-5 was shown only in girls. In the final model, only PiL was found to be a significant predictor of FOMO, while gender, grade, WHO-5 and FAS were rejected. Conclusions: Our study has some practical implications, such as how to avoid problematic social media use. Classes devoted to the meaning of life and the place in life should be organised.