In the United States, most adolescents do not obtain the recommended amounts of physical activity for optimal health. Around 80% of adolescents own a mobile device, and social media is frequently used by adolescents on mobile devices. Few studies have examined the use of social media as part of an intervention to promote physical activity. The purpose of this study was to evaluate the use of a Facebook group as part of a mHealth physical activity intervention trial. Adolescents, ages 14-18 years, were recruited for a four week physical activity intervention using the FitBit Flex. Participants were also given the option to join a private Facebook group where they could interact and were given badges for fitness accomplishments. The research assistant moderator posted on the Facebook group an average of 25.3 times (SD=7.2). Post-intervention, participants completed a phone interview about their experience. Of 30 intervention participants (avg age 16.0 (SD=1.1), 60.0% female), 17 opted to join the Facebook group (avg age 16.3 (SD=1.2), 47.0% female) of which 10 completed a qualitative interview. Participants averaged 4.9 interactions (SD=8.7) on the Facebook group wall throughout the intervention. From the interview responses, major themes included enjoying the badge feature of the Facebook group and wanting more content and interaction. In conclusion, participants used and enjoyed having the Facebook group, particularly the badge feature of the group, as an adjunct to the physical activity intervention.