Virtual online communities (social networks, wikis. . . ) are becoming the major usage of the web. The freedom they give to publish and access information is attracting many web users. However, this freedom is filling up the web with varied information and viewpoints. This raises important issues that concern privacy and trust. Due to their decentralised nature peer-to-peer (P2P) systems provide a partial solution for the privacy problem: each user (peer) can keep control on her own data by storing it locally and by deciding the access they want to give to other peers. We focus on semantic P2P systems in which peers annotate their resources (documents, videos, photos, services) using ontologies.