Young adults are increasingly integrating social media into their daily lives and social behavior, and empirical work on the personality traits and interpersonal competency associated with use and the potential consequences of use on social behavior is still emerging. The present study sought to investigate the interrelationships among constructs relevant to the developmental tasks associated with emerging adulthood and Facebook use, including adult attachment style, Five-Factor Model personality traits, and interpersonal competency. Using data collected from 617 emerging adults, we used structural equation modeling to test a theoretical model explaining interrelationships among the constructs under study. Results from our data yielded a well-fitting model indicating that insecure attachment had direct and positive effects on neuroticism, direct and negative effects on extraversion, direct and negative effects on interpersonal competency, and negative indirect effects on Facebook use. In addition, only extraversion was related to interpersonal competency and Facebook use when first accounting for attachment style. These results highlight the role of attachment style, and its importance in both developing personality traits and interpersonal skills, and online social behavior, which aligns well with an attachment theory framework. Lastly, we discussed future directions for research, as well as theoretical and practice implications for psychologists.