Online networks like email, Facebook, LinkedIn, Wikipedia, eBay, and BitTorrent-like Peer-to-Peer (P2P) systems have become popular and powerful infrastructures for communication. They involve potentially large numbers of humans with their collective inputs and decisions, and they often rely on the cooperation and the contribution of their users. Nevertheless, users in online networks are often found to be selfish, strategic, or even malicious, rather than cooperative, and therefore they need to be incentivized for contributions. This thesis provides theoretical and practical insights into the correlation between user behaviors and contribution incentives in online networks. It contains a demonstration of user behaviors and their consequences at both the system and the individual level, an analysis of barter schemes and their limitations in incentivizing users to contribute, an evaluation of monetary schemes and their risks in causing the collapse of the entire system, and an examination of user interactions and their implications in inferring user relationships.