Brief Report Emerging Adults’ Public and Private Discussions of Substance Use on Social Media Madeleine J. George 1 , Samuel E. Ehrenreich 2 , Kaitlyn Burnell 3 , Allycen Kurup 1 , Justin W. Vollet 3 , and Marion K. Underwood 1 Abstract Substance use increases during emerging adulthood and may relate to higher concurrent and future problems. For a community sample of 140 emerging adults, this study explores the associations between reported alcohol, marijuana, and tobacco use in 12th grade, the content of public posts and private messaging on Facebook during the fall after 12th grade, and self-reported substance use 1 year after high school. About one quarter of participants discussed substances publicly, and nearly half discussed substances privately on Facebook as observationally coded by researchers. Twelfth-grade substance use predicted the probability of engaging in public and private substance-related discussions. Tobacco and marijuana use predicted the frequency of private messaging about substances. Public and private online substance discussions predicted positive changes in marijuana use 1 year later. Results from this study suggest that social media discussions about substances, particularly private messages, may signal and shape emerging adults’ substance use behaviors. Keywords social media, substance use/abuse, alcohol use/abuse, longitudinal, technology Substance abuse costs the United States (US) an estimated $740 billion annually in health care, work loss, and crime (National Institute on Drug Abuse, 2017). Substance use is linked to poor health and well-being (Hall et al., 2016), and usage rates steadily increase during emerging adulthood (Chen & Jacobson, 2012; Schulenberg et al., 2018). Emerging adulthood is defined as the developmental period from ages 18 to 25 that encompasses the transition to adulthood (Arnett, 2000). During this transition, individuals gain independence and may have more opportunities to experiment with sub- stances (Arnett, 2005). It is imperative for research to exam- ine factors related to emerging adults’ substance use, but emerging adults may be reluctant to report on illegal beha- viors and/or may use substances in contexts that are difficult to observe. Social media may provide a new context to assess emerging adults’ discussions about substances. This longitu- dinal study investigated whether the observed content of emerging adults’ public posts and private messages on Face- book was associated with self-reported alcohol, marijuana, and tobacco use in a community sample during the emerging adulthood transition. Prevalence and Correlates of Substance Use During Emerging Adulthood Large national surveys of U.S. college students find that, on an average day, 13% of college students drink alcohol and 8% use marijuana (Lipari & Jean-Francois, 2016), and, over the previ- ous year, many college students used alcohol (76%), marijuana (38%), and tobacco (17%; Schulenberg et al., 2018). Although findings vary in part due to measurement challenges, studies suggest that emerging adults who are White, male, and not attending college are more likely to use substances than emer- ging adults who are non-White, female, and college enrolled 1 Purdue University, West Lafayette, IN, USA 2 University of Nevada, Reno, Reno, NV, USA 3 University of Texas at Dallas, Richardson, TX, USA Corresponding Author: Madeleine J. George, PhD, Psychological Sciences, Purdue University, PRCE HALL Suite 385, 701 3rd Street, West Lafayette, IN 47907, USA. Email: georg107@purdue.edu Emerging Adulthood 1-7 ª 2019 Society for the Study of Emerging Adulthood and SAGE Publishing Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/2167696819867533 journals.sagepub.com/home/eax