Computing, 2016: What Won’t Be Possible? - New York Times: "Social networks, noted Jon Kleinberg, a professor at Cornell, are pre-technological creations that sociologists have been analyzing for decades. A classic example, he noted, was the work of Stanley Milgram of Harvard, who in the 1960’s asked each of several volunteers in the Midwest to get a letter to a stranger in Boston. But the path was not direct: under the rules of the experiment, participants could send a letter only to someone they knew. The median number of intermediaries was six — hence, the term “six degrees of separation.” "
I'd heard of the "Six Degrees of Separation" idea for a long time, but I never knew the origins.