Background/Objectives: Depression is a leading contributor to global disease burden and increasingly linked to social media use. While most evidence associates high use with depressive symptoms in adolescents and young people, adult populations remain understudied. Beyond exposure time, how and why social media are used may be critical to understanding this association.

Methods: A cross-sectional online survey was conducted among 849 adult Instagram and TikTok users. Participants completed sociodemographic questionnaires and the Patient Health Questionnaire (PHQ-9). A Harmful Social Media Use Score (0-3) was constructed based on daily use > 2 hours, passive use, and negative pre-use emotional state. Associations with depression severity were analysed using multivariable adjusted ordinal logistic regression.

Results: A clear dose-response relationship was observed between the Harmful Use Score and depression severity. Each additional point increased the odds by 78% (aOR = 1.78 (95%CI: 1.49-2.12)). Compared with score 0, participants with score 3 showed an odds ratio of 4.93 (95%CI: 2.61-9.33) of more severe depressive symptoms. Among score components, negative pre-use emotional state showed the strongest association (aOR = 2.92), followed by daily use > 2 hours (aOR = 1.66). Results were consistent across sex, age, and having a previous diagnose of a mental disorder.

Conclusions/Recommendations: Combined emotional and behavioural patterns of social media use are strongly associated with depression severity in adults. Stratification by composite risk levels identifies vulnerable profiles and may support early detection and preventive strategies in digital mental health.