Background: Older adults are at a high prevalence of depressive symptom. Previous work has revealed that Internet usage was significantly associated with depressive symptoms. However, the mediating mechanism underlying this relationship has been underexplored. This study aims at examining the potential mechanisms—social networks and social adaptation—through which Internet usage impacts depressive symptoms and exploring the urban–rural heterogeneity among Chinese older adults. Methods: Data were sourced from the 2018 China Longitudinal Aging Social Survey, which provided a nationally representative sample of participants aged 60 and above (N = 9895). Structural Equation Modeling (SEM) was conducted to investigate the chain-mediating effect between friendship/family networks and social adaptation in the association of Internet usage and depressive symptoms. Results: (1) After controlling for covariates, Internet usage was directly associated with a lower level of depressive symptoms. (2) Depressive symptoms were partly influenced by Internet usage through three distinct pathways: the mediating roles of family and friendship networks, the mediating role of social adaptation, and the serial mediation of both friendship networks and social adaptation. (3) Urban–rural differences were observed within the aforementioned serial mediation models. Limitations: This cross-sectional study prevented us from causal inference. Conclusions: This study highlights the serial mediating role of social networks and social adaptation in the connection between Internet usage and depressive symptoms. Policy interventions for older adults should incorporate digital technology to facilitate Internet usage, with rural elders identified as a priority target population.