Using survey data collected from 260 children, adolescents, and young adults between the ages of 9 and 26, this paper offers evidence for a relationship between social networking site use and Imaginary Audience, a developmental variable in which adolescents believe others are thinking about them at all times. Specifically, after controlling for a number of variables, results indicate a significant, positive relationship between social networking site use and Imaginary Audience ideation. Additionally, results indicate a positive relationship between Imaginary Audience ideation and Facebook customization practices. Together, these findings provide evidence, based on Vygotskian developmental theory, for a general consideration of the role that currently available tools, in this case social networking sites, can have on development. Thus, findings implicate both the role of development on social networking site use, as well as the role of social networking site use on development. Overall, these findings have important implications for the study of media and human development, which are discussed in detail.