LinkedIn appears to build "social graphs" based on profile views. If person A visits the profile of person B and C it is quite likely that some form of relation

LinkedIn appears to build "social graphs" based on profile views. If person A visits the profile of person B and C it is quite likely that some form of relationship exists (perhaps an inverse relationship too). It doesn't just have to be profile views -- LinkedIn most likely keeps track of search terms too. If a random unknown visitor comes along and searches for both "Person A" and "Person C" then it is likely that a connection exists between these two persons. It would be easy to train this system by displaying a "guess" (a friendship recommendation for person A) to persons B and C. If person B or C show interest in the recommendation then perhaps a relationship exists. Guesses could also be formed by comparing keywords/metadata found on profiles in two different social circles. The reason I mentioned LinkedIn is that I think they do a better job of recommending/guessing who your acquaintances are than Facebook.