User segment population techniques

A facility for identifying users that have an interest in a selected subject is described. For each of a number of actual users, the facility establishes a user behavior record describing the behavior of the actual user. The facility also establishes a user behavior record describing behavior of a virtual user. The behavior described by the user behavior record for the virtual user is made up of actions each tending to indicate an interest in the selected subject. The facility identifies a proper subset of the actual users whose user behavior records are most similar to the user behavior record of the virtual user as interested in the selected subject.