Psychology Today: Six Degrees: Urban Myth?:
Milgram's small-world experiment took this idea a step further: His subjects could reach anyone in the country, maybe anyone on the planet, through a chain averaging just a few people.
In the intervening decades, Milgram's findings have slipped away from their scientific moorings and sailed into the world of imagination. The "six degrees of separation" between any two people has been adopted by the intelligentsia, and it has turned up in the media, movies and on Web sites.
But Milgram's startling conclusion has scanty evidence. The idea of six degrees of separation may, in fact, be plain wrong—the academic equivalent of an urban myth.