Building on Junco’s (2013) study examining the accuracy of self-reported computer-specific time on Facebook, the current study investigates the accuracy of self-reported time on multiple social media (SM) platforms across multiple electronic devices and evaluates whether reporting accuracy is systematically associated with participant sex, individual SM platform in question, or total number of SM platforms used. Participants were 320 college students who downloaded software on their computers, tablets, and smartphones to track their active use of Facebook, Twitter, Instagram, and Snapchat over a 2-week surveillance period and then self-reported their daily average minutes on each platform immediately after. Larger proportions of students over- estimated than under-estimated their use, with the largest overestimations found on Snapchat and Instagram. Relative to males, females logged significantly more SM time and were less accurate in reporting. Overall, the likelihood of substantial inaccuracies in reporting total SM time and time on most individual platforms increased with each additional SM platform participants reported using. Findings from this study cast further doubt on the validity of self-report SM measures in the present SM landscape and underscore the need for either data analytic strategies to adjust for systematic reporting biases or a shift towards objective time-tracking methods.