1. A computer-implemented method comprising: storing, in computer storage, event data representing user-generated events that reflect user affinities for particular items, wherein the user-generated events comprise user actions that can be monitored by a computing device; generating a score that reflects a degree to which item affinities of a first user of a plurality of users are similar to item affinities of a second user of said plurality of users, said score taking into consideration a first plurality of user-generated events by the first user reflecting a set of item affinities of the first user, and a second plurality of user-generated events by the second user reflecting a set of item affinities of the second user, wherein generating the score further comprises accessing item similarity data to determine whether an item in the set of item affinities of the first user is similar to an item in the set of item affinities of the second user; and based at least in part on the score, determining whether to output to the first user information about the second user.