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