The design of incentive schemes for P2P multimedia sharing networks is challenging due to the unique features exhibited by such networks: large populations of anonymous peers interacting infrequently, asymmetric interests of peers, network errors, and multiple concurrent transactions. In this paper, we design and rigorously analyze a new family of incentive protocols that utilizes social norms for multimedia sharing. In particular, we show that, given the network and peers' characteristics, social norms can be effectively designed to deter free-riders by reducing their reputations and thus, the services which they receive from the P2P network. Unlike existing research, which deploys ad- hoc reputation schemes in P2P networks, our proposed framework rigorously determines the optimal social norm and associated reputation scheme to be used by a particular P2P system, characterized by its specific network characteristics. We also investigate how the design of the optimal social norms needs to change to account for the impact of altruistic and malicious peers. Our results show that optimal social norms are capable of providing significant improvements in the sharing efficiency of multimedia P2P networks. Specifically, depending on the network environment, the proposed social-norm based P2P protocols are able to outperform conventional Tit-for-Tat protocols by up to 8dB in terms of video quality.