Les réseaux sociaux d'aujourd'hui. Un monde décidément bien petit

In a famous experiment, Stanley Milgram showed in 1969 that, on the average, 6 links (therefore passing through 5 intermediates) were sufficient to connect two strangers in the United States. These six degrees of separation have been observed by other studies but also carrying on small samples. The advent of social networks on the Internet has recently allowed checks on a massive scale. Studies of the Microsoft instant messaging, Twitter or Facebook confirmed and even amplified the small world hypothesis. Thus, by analyzing the 69 billion relationships between the 721 million people having logged into Facebook in May 2011, it appears that the average distance (or number of degrees) between two randomly selected users on the planet is 4.7. Various explanations of the small world are discussed in this article (weak ties between clusters, the presence of hubs in a scale-free structure) and appear in the end rather complementary. JEL classification : Z1, Z19