Abstract : Being a Community manager is an emerging employment in social Web companies. His or her role is to monitor communities on a devoted social website, in order to understand new trends or behaviours. He or she also has to discover and attract new potential users of the website in external resources like web forums, that are not necessarily on the same topics nor explicitly defined. In this paper we propose a scalable protocol to monitor on-line communications, like web forums, walls or twits. We provide an analysis method to extract implicit communities and user interests based on the semantics of data exchange and the structure of communications. The method is parameterized by a target vocabulary expressed as an ontology, in order to focus on relevant communities.