Let’s say we have a population of 1,000 people with 10 friends each and no “random” friends. That is, everyone’s friends are drawn only from a strictly defined social circle. Then the average degree of separation is 50; in other words, on average it will take 50 hops to get from one randomly selected person to another. But if we now say that 25% of everyone’s friends are random, that is, drawn from outside their normal social circle, then the average degree of separation drops dramatically to 3.6.
 Your collection of friends most likely includes people you grew up with, people you went to school with, colleagues from work, people in your profession, and your current neighbors. These are friends who come from your normal social circles.
 In addition to your structured social network, you also have a few random friends, people who are not in your normal social circle, who your have somehow met and become friendly with. For example, it might be someone you got to know while on vacation, or in the waiting room of a doctor’s office.
— The Origin of Wealth by Eric D. Beinhocker