“…The importance of referrals has been known for a long time, e.g., [Lin, 1971, Reynolds and Darden, 1971, Reingen and Kernan, 1986, but the ideas of referral systems appeared only recently, e.g., MINDS (based on the documents used by each user) [Bonnell et al, 1984, Mukhopadhyay et al, 1986, and ReferralWeb (based on co-citations, i.e., co-occurrence of names on WWW pages) [Kautz et al, 1997b,a]. The informal person-to-person social networks are used to construct "referral chains" of requests for expertise, and software agents help automate the search of information and expertise [Kautz et al, 1996].…”