This study examined the relationship between quality of friendship and social networking site (SNS) usage, co-rumination, and self-disclosure among 355 adolescents aged 11 to 15. Participants completed an online questionnaire addressing frequency, intensity, and rate of their online communication, how they perceived the positive and negative aspects of their peer relationships; and the methods with which they discussed problems with friends. Participants were also asked to list up to 10 individuals with whom they interacted most when using SNS and face-to-face to identify overlaps in their on- and off-line friendships. Findings showed that quality of friendship was positively correlated with co-rumination and self-disclosure, and that SNS usage was positively correlated with age and overlaps in online and offline friendship. Results indicated a positive effect of age and self-disclosure on quality of friendship. A main effect of gender was found whereby girls reported higher SNS usage, and levels of co-rumination, and self-disclosure than boys. Developmental differences were examined qualitatively through participants' reports of the overlap in friendship, in which participants ages 14 to 15 showed greater overlap in their online and offline relationships when compared with participants ages 11 to 13. SNS usage served as a predictor of the level of overlap found between online and offline friendships. Collectively, these findings emphasize the importance of understanding SNS usage in adolescence, as peer relationships and internet usage continue to increase in importance with age. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]