A computer-implemented method is provided, which includes constructing first and second user profiles for respective first and second users, by observing respective online behaviors of the first and second users. The first and second user profiles are projected onto an N-dimensional vector space as respective first and second vectors. An interaction between the first and second vectors in the vector space is modeled. Responsively to the modeled interaction, at least one entity is modified, the entity selected from the group consisting of: the first user profile, and the first vector. Content is presented to the first user responsively to the modified entity. Other embodiments are also described.