1. A method performed by a computing system including a processor and a memory for identifying users among a plurality of users that have an interest in a selected subject, comprising: for each of a plurality of actual users, establishing a user behavior record describing behavior of the actual user; establishing a virtual user behavior record describing behavior of a virtual user, the behavior described by the virtual user behavior record comprising actions each tending to indicate an interest in the selected subject, wherein the virtual user behavior record is not based on behavior of an actual user; and identifying as interested in the selected subject a proper subset of the plurality of actual users whose user behavior records are most similar to the virtual user behavior record established for the virtual user, wherein the identification is performed by applying a cohort matching technique to the user behavior records for the actual users and the virtual user, and wherein the applied cohort matching technique comprises: determining, for each of the user behavior records established for an actual user, a distance between (a) the behavior described by user behavior records established for the actual user and (b) the behavior described by virtual user behavior records established for the virtual user; and identifying users among the plurality of actual users having the smallest distances from the virtual user.