Six Degrees of Separation
https://www.theguardian.com/technology/2008/aug/03/internet.email
The nature of social networks to contain many short paths is called the small-world phenomenon, or “six degrees of separation”. This theory was first developed by a study by social psychologist, Stanley Milgram, who assigned experiment participants to forward a letter to a target living in Sharon, MA, knowing only the target’s name, address. A third of the letters reached the target, averaging six steps.
Since then, the idea of six degrees of separation has been contended with in several studies. In 2008, Microsoft performed a study, observing the records of 30 billion electronic conversations among 180 million people from multiple countries in the first planetary-scale social networks study of its kind. The study involved all Microsoft Messenger instant messages in the month of June 2006 (accounting for half the world’s instant-messaging traffic during the period). The researchers qualified all pairs of participants who sent each other messages as acquaintances, and found that 78% of all pairs were connected in seven steps or fewer, averaging 6.6 connections for any random pair to meet — although some pairs were separated by even up to 29 steps.
In 2003, Columbia University researchers also used the internet in an attempt to pursue the claim of six degrees of separation. 24,000 volunteers were asked to target emails to one of 18 random people from 13 possible countries. Ultimately, only 384 of these emails reached the targets, averaging between five and seven steps.
Trials have been completed using networks such as Facebook as well, but it is difficult to use media such as Facebook and Microsoft Messenger to confirm the theory since most people using these services come from similar socioeconomic levels.