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In peer-to-peer networks (P2Ps), many autonomous peers without preexisting trust relationships share resources with each other. Due to their open environment, the P2Ps usually employ reputation systems to provide guidance in selecting trustworthy resource providers for high reliability and security. However, node collusion impairs the effectiveness of reputation systems in trustworthy node selection. Although some reputation systems have certain mechanisms to counter collusion, the effectiveness of the mechanisms is not sufficiently high. In this paper, we leverage social networks to enhance the capability of reputation systems in combating collusion. We first analyzed real trace of the reputation system in the Overstock online auction platform which incorporates a social network. The analysis reveals the important impact of the social network on user purchasing and reputation rating patterns. We thus identified suspicious collusion behavior patterns and propose a social network based mechanism, namely Social Trust, to counter collusion. Social Trust adaptively adjusts the weight of ratings based on the social distance and interest relationship between peers. Experimental results show that Social Trust can significantly strengthen the capability of current reputation systems in combating collusion.