Focusing on fitness app use and social context of fitness postings on social media, this study examined the implications of mHealth technologies use for fitness. This study explored descriptive information about respondents’ use of fitness apps such as self-monitoring, self-regulation, social facilitators, and rewards. Furthermore, respondents’ fitness posting experience was also explored. For respondents who saw others’ fitness posts, this study examined how viewers’ social comparison on fitness postings (upward and downward) related to their physical activity (PA) self-efficacy, motivation, and participation. For those who posted about their fitness information on social media, this study investigated fitness posters’ ways of self-presentation related to receiving supportive feedback, and how supportive feedback related to fitness posters’ PA motivation and participation. This study recruited fitness app users from a crowdsourcing internet marketplace. Quantitative data analysis examined the role of social comparison, self-presentation, and supportive feedback in respondents’ PA self-efficacy, motivation, and participation. The results revealed that people mostly used the fitness apps for physical activity-related self-monitoring and self-regulation. For those who engaged in upward social comparison tended to have more self-efficacy for PA, PA motivation, and therefore participated more in PA. Both positive and negative self-presenters received more supportive feedback from others. The more supportive feedback fitness posters received, the more self-efficacy for PA they had. The more self-efficacy for PA fitness posters had, the more PA motivation they had. The results also showed that people received more esteem support and emotional support from others when they positively presented their fitness on social media. Fitness posters with negative self-presentation received more emotional support and informational support.