Methods, systems, and apparatus, including computer programs encoded on computer storage media, for a collective address book system. One of the methods includes: receiving, at the system, a request from a user regarding a topic; selecting automatically, at the system, a potential referrer, the potential referrer selection being based on the likelihood that the potential referrer will respond to the user's request; and communicating automatically with the potential referrer regarding the user's request without the system indicating to the user that the potential referrer has been contacted.