The six degrees of separation comes partly from an experiment that was done in the 60s.
People were sent letters describing someone and asked to give that person the letter - if they did not know the person, they should pass the letter to the person they knew that they thought was most likely to know them. The number of steps was measured.
Great experiment read about it here:
http://en.wikipedia.org/wiki/Small_world_exper iment
However there are problems with it - this was just done in the US. A lot of chains will never be completed and you may have some completely isolated people - I don't think any were addressed to Amish for example.
It was also before the Internet which has changed peoples networking quite considerably