Wired has a brief interesting discussion on network space:

A fascinating (and deceptively simple) example of network space is the small-world phenomenon, the idea that anyone on the planet can be connected to anyone else through just six degrees of separation. Although the notion had been floating around in popular culture for much of the 20th century, it was only tested in the late-'60s in an experiment conducted by social psychologist Stanley Milgram. Milgram gave letters to about 300 people in Boston and Omaha with instructions that the envelopes ultimately reach a single "target," a Boston stockbroker. The letters could be sent only to a personal friend of the current holder, who then received the same instructions. To the surprise of many, more than 60 of the letters reached their goal, changing hands, on average, only six times. Milgram's conclusion was that people who seem very distant in physical or social space may actually be closer than we imagine.