Social media can be an effective tool to foster values and lifestyles such as physical activity. However, it is still largely unknown what type of social media information helps to promote physical activity practices. This research aims to (1) identify the combination of variables that explain social media engagement in physical activity (SME-PA) and its relationship with physical activity behavior and (2) determine what type of social media content posted on social media could be most effective in promoting physical activity behaviors. Using a quasi-experimental and an experimental study, the findings showed that the conditions of high levels of information availability, social ties, trust in social media information, and opportunity seeking were present and combined in three of the four solutions (77% of the cases). SME-PA was correlated with the intention to improve physical fitness, and the physical shape, the type of benefit associated with physical activity practice and the gender of the person in the photo practicing physical activity were found to be relevant variables for improving the physical activity-related behavior of social media users. Finally, several implications are proposed to improve the administration of social media content for SME-PA.