The "Kevin Bacon" theory suggests that there are just "six degrees of separation" between any two people on Earth

This is a theory which has been largely corroborated by a huge Microsoft study of electronic communication. This was all done be reviewing 30 billion electronic conversations among 180 million people from around the world, or roughly half the world's instant-messaging traffic at that time!

"To me, it was pretty shocking. What we're seeing suggests there may be a social connectivity constant for humanity," said Eric Horvitz, a Microsoft researcher who conducted the study with colleague Jure Leskovec. "People have had this suspicion that we are really close. But we are showing on a very large scale that this idea goes beyond folklore."

Instant-Messagers Really Are About Six Degrees from Kevin Bacon - washingtonpost.com