In recent years, there has been a phenomenal proliferation of multifunctional fitness apps (MFAs), many of which are deeply ingrained into modern daily life as an aid to trace, manage, and improve users’ health and fitness. With technological advancement, these emerging information technology (IT) artifacts have the potential to facilitate the sustainable development of society and the environment. However, MFAs are facing a critical predicament: how to ensure user stickiness and continuous use. Drawing on the social support theory, this study aims to investigate stickiness as a multi-dimensional construct and to explore the antecedents of continuance usage and continuance purchase intentions in MFAs. The research model is empirically tested with data collected from 328 users of mobile fitness apps. A structural equation modeling analysis reveals both instrumental stickiness and social stickiness exert statistically significant influences on continuance usage intentions and in-app purchase intentions. Appraisal support, information support, and network support are significant antecedents for instrumental stickiness. While emotion support, esteem support, network support, and tangible support are significant antecedents for social stickiness. In addition, two facets of user sustainability—instrumental stickiness and social stickiness—exert different levels of influence on continuance usage and continuance app purchase intensions, respectively. The findings of this study will provide valuable insights for practitioners and policymakers involved in designing sustainable health products and engendering e-health systems.