The rapid growth of network access and the development of Web 2.0 have resulted in the popularity of virtual communities (VCs), such as Wikipedia, Facebook, and LinkedIn. Although these online communities provide no monetary incentive for sharing knowledge, as most businesses will do, they still become a popular platform for knowledge sharing. The purpose of this study is to investigate the motivation for people to contribute in virtual communities, whether their motivations differ in different types of virtual communities, and whether their behavior and motivations will be affected by monetary reward. Social exchange theory and self determination theory were adopted to explore the relationships between behavior and motivations of virtual community members. Motivation crowding theory was used to examine the effect of monetary reward on knowledge sharing. The research framework includes two types of motivators (intrinsic and extrinsic), two different virtual community types (common identity vs. common bond), and one treatment (monetary reward). An online survey and an experiment with monetary incentive were conducted on two virtual communities: ITToolbox and LinkedIn. The results showed that there were significant moderating effects between these two types of virtual communities. A strong positive relationship was found between intrinsic motivation and knowledge sharing for the professional community (common identity). In contrast, the relationship between extrinsic motivation and knowledge sharing was stronger in the social networking community (common bond). The results also confirmed the existence of the crowding effect that the intrinsic motivation was significantly declined when an extrinsic monetary reward was provided.