1. A computer-implemented method comprising: constructing first and second user profiles for respective first and second users, by observing respective online behaviors of the first and second users; projecting the first and second user profiles onto an N-dimensional vector space as respective first and second vectors; modeling an interaction between the first and second vectors in the vector space; responsively to the modeled interaction, modifying at least one entity selected from the group consisting of: the first user profile, and the first vector; and presenting content to the first user responsively to the modified entity.