14. The system of claim 9 , wherein generating the score comprises: identifying items of a first type in common between the set of item affinities of the first and second users; identifying items of a second, different type in common between the set of item affinities of the first and second users; and weighting each of the identified common items as a function of the respective type of the identified common item.
9. A system comprising: memory for storing 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; and at least one computing device including a processor in communication with the memory, the at least one computing device operative to: generate 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, determine whether to output to the first user information about the second user.