I'd actually venture to say that Facebook's 'friends of friends' algorithm isn't all that bad. It seems to be loosely based on the idea of triadic closure (except there is only a single kind of tie). If 'A' is friends with 'B' (you) and friends with 'C' (them), it is often natural for a friendship to form between 'B' and 'C' due to shared interest in 'A'. Obviously this property is a bit extreme and over-simplified to hold in large networks, but it is a decent heuristic. I think if Facebook included filtering by location and other strong features then their algorithm would be much better. http://en.wikipedia.org/wiki/Triadic_closure