Why are there six degrees of separation in a social network?

In the short story "Chains" (1929), the Hungarian writer Frigyes Karinthy described a game where a group of people was discussing how the members of the human society were closer together than ever before. To prove this point, one participant proposes that any person out of the entire Earth population (around 1.8 billion at that time) could be reached using nothing except each personal network of acquaintances, betting that the resulting chain would be of no more than five individuals. The story coined the expression `six degrees of separation' to reflect the idea that all people of the world are six or fewer social connections apart from each other.