1. A computer-implemented method comprising: constructing, by a computer system, first and second user profiles for respective first and second users, by observing respective online behaviors of the first and second users; projecting, by the computer system, the first and second user profiles onto an N-dimensional vector space as respective first and second vectors; modeling, by the computer system, an interaction between the first and second vectors in the vector space, during which modeled interaction both the first vector moves closer to the second vector and the second vector moves closer to the first vector in the vector space; responsively to the modeled interaction, modifying, by the computer system, the first user profile; and presenting content to the first user responsively to the modified first user profile.