Young African Americans are heavy users of social networking technologies. In 2013, 96% of African Americans aged 18 to 29 used a social networking site of some kind. Understanding how online social networks facilitate behavior change can bridge important gaps in the way technology can be used to intervene on health among underserved populations. The primary objective of this study was to test the efficacy of a mobile app (PennFit) intervention in increasing participants' daily active minutes objectively recorded by a fitness tracking device (Fitbit zip). In the control group, participants used the PennFit app to record and monitor their own physical activity progress. In the online social network intervention, participants were randomized to 4-women networks and were able to see and compare their own recorded physical activities with activities of the other three women in their network. Participants in a network had access to an online chatting tool to chat with one another. The secondary objective was to understand the intervention's mechanisms through mediation analysis on theoretical variables.