A growing literature attests to the associations between social media use (SMU) and psychosocial outcomes. However, recent meta-analyses and systematic reviews indicate relatively small and inconsistent effect sizes across studies. Such reviews also indicate that measurement approaches have focused almost exclusively on self-report methodologies to assess SMU, raising concerns about the role of common method variance. The current study was designed to examine the concurrent validity of self-reported SMU, the degree to which objective SMU estimates fall within the categorical ranges indicated by the self-report data, and the degree to which objectively reported SMU explains psychosocial functioning beyond that accounted for by self-reported SMU in a sample of adolescents (n = 317) and young adults (n = 325). Results indicate small but significant associations between self-reported SMU and objective minutes of SMU (r = .25), number of pick-ups (r = .13), and number of notifications (r = .12). For approximately one-third (34.6%) of the total sample, objectively reported SMU fell within the range indicated by self-report categorical data. Slightly more participants (42.6%) overestimated their SMU relative to objective data. Finally, for adolescents only, objective SMU variables (pick-ups and notifications) was associated with self-reported depressive symptoms and social role functioning after accounting for self-reported SMU estimates. Overall, results support the use of self-reported SMU estimates, particularly in research designs that rely on correlational methods to examine associations between SMU and psychosocial outcomes. However, results also suggest that both adolescents and young adults significantly under- and over-report SMU relative to objective data.