Ghost of a flea

One of the cardinal rules of human networks is "Birds of a feather flock together". Friends of friends become friends, and coworkers of coworkers become colleagues. Dense clusters of connections emerge throughout the social space. The usual pattern found throughout social structures[and many other complex systems] is dense intra-connectivity within clusters with sparse inter-connectivity between clusters.

One day, while searching for a book on amazon.com, I started thinking about Amazon's value-added service -- Customers who bought this book also bought these books. Amazon lists the top 6 books that where bought by individuals who also bought the book currently being browsed. I wondered...