As HCI researchers have designed tools to promote wellness, disease has often been approached as a general problem. In contrast, public health research argues for an activist approach focused on how certain groups disproportionately experience disease and eliminating these disparities. Taking this activist stance, we examine how technology can reduce health inequalities by disrupting power relationships and helping communities pursue social change. We discuss our tool, Community Mosaic (CM), which allows individuals to share their healthy eating ideas with one another as a means of advocating behavior change. Our results characterize how CM helped facilitate activism (i.e., collective efforts to counter local challenges to healthy living) and shift users' attitudes regarding their role as advocates for health. We contribute to the field of HCI by using our findings to present a set of recommendations for future research focused on designing and evaluating health promotion tools using an activist lens.