Health-related online social networks are starting to play a role in many people's daily lives by enabling them to monitor their diet and motivating them to change their lifestyles. These social networks provide different motivation mechanisms. However, little research has been done on the effectiveness of these motivation mechanisms. This research analyzes data collected from a digital health community to examine what mechanisms can help motivate people. The results suggest that there is a high level of correlation between users' exercise activities and their participation in these digital health communities. This research benefits the digital health communities by providing insights into the design of motivation mechanisms.