1. A computer-implemented method for determining similar users, comprising: receiving information for a source user, at a computer system, the information including at least one topic and a user value for each topic, where: the user value includes a user authority value representing a user expertise related to that topic and a user interest value representing a degree of user association with that topic, and the user value represents how strongly the user is associated with that topic; generating similarity scores based on a user value for each topic for the source user and a user value for the same topic for each user in a set of users, where each user in the set of users is associated with a user value for each topic; selecting one or more similar users based on the generated similarity scores; outputting one or more of the selected users; and outputting the identity of one or more overlapping topics and a value indicating a degree of overlap for each overlapping topic.