Recent years have seen growing concern about the detrimental effects of digital activities on well-being and mental health. A major axis of concern is problematic internet use (PIU): dysregulated or excessive uses of online websites or applications, which can interfere with other daily-life activities. Given potential health impacts, better understanding of the predictors of PIU is warranted. This study investigated possible socio-demographic and psychological determinants of PIU for various types of Internet activities. We recruited a sample of 1504 participants representative of the French population aged 15 and above. Socio-demographic variables (sex, age, income, educational attainment) and psychological traits (dispositional flow, trait and state anxiety, fear of missing out) were surveyed, along with problematic uses in major digital activities (gambling, streaming, social media, video game, shopping, news, cybersex). The results showed that fear of missing out had the most explanatory power for PIU, followed by anxiety. Contrary to our expectations, socio-demographic variables explained very little variance in PIU, with only age emerging as a weak predictor in some analyses. Overall, psychological traits appeared to be more promising predictors of problematic uses, with detrimental effects of fear of missing out and anxiety presumably reflecting maladaptive coping strategies or unfulfillment of psychological needs. These results open up discussion regarding the design of digital platforms and public mental health policy.