Background: Research on the relationship between Facebook use intensity and depressive symptoms has resulted in mixed findings. In contrast, problematic Facebook use has been found to be a robust predictor of depressive symptoms. This suggests that when intense Facebook use results in a problematic usage pattern, it may indirectly predict depressive symptoms. However, this mediation pathway has never been examined. Moreover, it remains unclear whether the possible indirect relationship between Facebook use intensity and depressive symptoms through problematic Facebook use is moderated by demographic (age), and personality (neuroticism and extraversion) characteristics. Methods: To address these gaps, we conducted an online cross-sectional study (n = 210, 55% female, age range: 18–70 years old, Mage = 30.26, SD = 12.25). We measured Facebook use intensity (Facebook Intensity Scale), problematic Facebook use (Bergen Facebook Addiction Scale), depressive symptoms (Center for Epidemiologic Studies Depression Scale Revised), and neuroticism and extraversion (Ten Item Personality Inventory). Results: A mediation analysis revealed that problematic Facebook use fully mediates the relationship between Facebook use intensity and depressive symptoms. Moreover, a moderated mediation analysis demonstrated that this indirect relationship is especially strong among young users and users scoring high on neuroticism. Conclusions: These findings expand our understanding of the mechanisms underlying the relationship between Facebook use intensity and depressive symptoms and describe user characteristics that act as vulnerability factors in this relationship.