In today's hyper-competitive business environments virtual organisations are becoming highly dynamic and unpredictable. Individuals may want to work together across organisation boundaries but do not have much prior knowledge of others. The semantic web and its associated new standards appear very promising as candidates to support a new generation of virtual organisations. In this paper a behaviour based organisation, Social Behaviour Networks, is proposed. In order to sense the changes on the web this paper focuses on a virtual sensor for allocating tasks amongst agents based on the announcements of tasks and capabilities of agents in DAML (DARPA Agent Markup Language). Due to the autonomy of agents the announcements are often vague and in a very high dimensional space. The ontology can provide useful information for achieving variable-resolution sensing from an individual agent's perspective and reducing the dimensions of the virtual space. The variable-resolution virtual sensors are based on hierarchical clustering analysis to reveal the level of similarity of announcements in the web.