A primary goal of this study was to examine adolescents' use of the most commonly used social media apps in the United States. An additional goal was to examine the relationship between social media use and adolescent externalizing behaviors. The moderating influences of gender and race/ethnicity also were assessed. The sample included 1,072 adolescents (M age = 12.74 years, SD = 0.67, 51% girls; 64% White). Participating adolescents completed a survey in school. Most of the adolescents did not use Facebook or Twitter. However, the majority of adolescents used Instagram and Snapchat, and girls were more likely to use these social media apps than were boys. Adolescents who regularly used social media reported more delinquent behavior and poorer behavioral conduct in comparison with nonusers. In addition, boys who used Facebook regularly and Hispanic adolescents who used Instagram regularly were at an elevated risk for delinquent behaviors and poorer behavioral conduct.