Although studies have investigated how the technical features of wearable fitness trackers promote physical activity, we understand less about how communication surrounding such devices may contribute to their success. Addressing current opportunities for inquiry is important, as the popularity of fitness trackers and other wearable devices grows. Through interviews with 25 people wearing fitness devices, this study elucidates how and with whom people communicate fitness tracker messages and explains the effects of sociomaterial practice on interactions between wearable fitness device users. Our findings show the vital role of communication in sharing and encouraging physical activity.