Despite the racial disparities in rates of chronic diseases and behaviors linked to chronic diseases, there have been relatively few RCTs of interventions to increase physical activity in African American women. Although some studies found significant improvement on physical activity, most focused on individuals and did not take into consideration the social contexts in which the participants' behaviors occurred. 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.