The impact of social media (SM) use (‘screentime’) on adolescent mental health has been the focus of increasing concern, despite mixed findings from empirical research. Current methodological approaches rely on self-reported SM use, which has limited accuracy and obscure the dynamic interplay of SM use and mental health. Smartphone-based mobile sensing offers new opportunities to gain insights into adolescents’ SM use patterns and behaviors, particularly at an idiographic level. Considerations and challenges of smartphone sensing methods for capturing adolescents’ SM use patterns and behaviors in clinical psychological science are discussed in the context of a pilot study using smartphone-based sensing with adolescents. The pilot study included 19 adolescents (Mean age = 15.84; 68% boys; 79% White) who installed a passive monitoring application (AWARE) on their phones for 31 (SD = 5.6) days. Descriptive data of sensing acceptability and feasibility are presented based on participant ratings and data yield ratio of usable data (74.18%). Sensing yielded 10,038 hourly observations collected from the ‘application foreground’ sensor across all participants from social media apps, and a total of 645 applications used. Categorization of SM apps were coded (kappa >.90) into ‘social networking’ (N = 20 apps) and ‘broader SM’ (N = 41) and compared to both Play Store-defined SM apps (N = 26) and popular SM apps based on Common Sense Media Survey (N = 9). Descriptive data on extracted behavioral features (duration, checking) from SM use categories (binned hourly and daily) are presented. Challenges, opportunities, and future directions of sensing methods for SM use are discussed to inform our understanding of its impacts on mental health and to improve the rigor of SM research in clinical psychological science.