With the continued advances of Web 2.0, health-centered Online Social Networks (OSNs) are emerging to provide knowledge and support for those interested in managing their own health. Despite the success of the OSNs for better connecting the users through sharing statuses, photos, blogs, and so on, it is unclear how the users are willing to share health related information and whether these specialpurpose OSNs can actually change the users' health behaviors to become more healthy. This paper provides an empirical analysis of a health OSN, which allows its users to record their foods and exercises, to track their diet progress towards weight-change goals, and to socialize and group with each other for community support. Based on about five month data collected from more than 107,000 users, we studied their weigh-in behaviors and tracked their weight-change progress. We found that the users' weight changes correlated positively with the number of their friends and their friends...