In a recently released study, Microsoft Research tracked all use of Mirosoft Messenger for June of 2006, and what they discovered was a far more connected world than you would expect.
According to Katharine Sanderson of Nature News, Microsoft logged every interaction on MSN Messenger in that month (all of it reportedly anonymous). They gathered 4.5 terrabytes of data, and found that most people were - you guessed it! - 6.6 steps away from another. In other words, very much like the old saying: everyone is separated by "six degrees." (The study also showed that the majority of users were between the ages of 15- and 30-years-old.)
The study is of course not perfect as it focuses only on Microsoft's own messenger, and none of the others such as AIM or GTalk. It's not difficult to imagine that if someone could find a way to do this same project with every IM system involved, you might find that the steps between individuals is even smaller.