Exercise tracking apps offer a novel and powerful channel to deliver behavioral interventions at a massive scale. As thousands of fitness tracking solutions emerge, the lack of systematic research quantifying their effectiveness on exercise outcomes becomes more prominent. Drawing on the motivation literature, this paper elucidates the effects of app-enabled motivation on fitness improvement. Specifically, this study examines the two most common forms of feedback available to users of exercise tracking apps, namely performance feedback and social feedback. The results of our field experiment reveal strong evidence that performance feedback and social feedback motivate fitness improvement. Furthermore, whereas the effect of social feedback diminishes over time, the effect of performance feedback remains resilient. There theoretical and practical implications of the findings are discussed.