The proliferation of the smartphones and wearable devices has given rise to the fitness app as one of the major app categories in current mobile app market. Drawing on the social cognitive theory, this study explores the impacts of fitness app functionalities on users' behavioral and psychological outcomes, in terms of exercise adherence and social engagement. Data from 267 college students at a public university in the United States indicated that personal-oriented functionalities of fitness apps can significantly improve both exercise adherence and social engagement of users; however, social-oriented functionalities can only help with perceived social engagement. Users' exercise proficiency level negatively moderates the relationship between personal-oriented functionalities and social engagement. Moreover, our findings show that perceived social engagement can encourage users' physical adherence to exercises. Implications and limitations of this study are discussed.