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This article aims to understand how the experts measuring online opinion address the general lack of demographic information about people talking on blogs, forums and social networks, whose conversations are analyzed. Facing the lack of any conventional variable, they must indeed find alternative collective forms to embody the trends of opinion they aim to unravel. Studying the case of Linkfluence (Paris, France), we show how these actors develop an alternative paradigm based on the concept of “online communities”, by which websites are categorized according to their hypertext profile and their theme. Therefore, the selection of theme-oriented communities shows a new and deliberately asymmetric way of sampling, that only takes into account mobilized and influential sites.