Founded in 2004 by a Harvard sophomore, Mark Zuckerberg, Facebook began life catering first to Harvard students and then to all high school and college students. It has since evolved into a broadly popular online destination used by teenagers and adults of all ages. In country after country, Facebook has cemented itself as the leader, often displacing other social networks.

Adding a new chapter to the research that cemented the phrase “six degrees of separation” into the language, researchers at Facebook and the University of Milan reported in November 2011 that the average number of acquaintances separating any two people in the world was not six but 4.74.

The research was based on a cohort of 721 million Facebook users. The researchers used a set of algorithms developed at the University of Milan to calculate the average distance between any two people by computing a vast number of sample paths among Facebook users. They found that the average number of links from one arbitrarily selected person to another was 4.74. In the United States, it was just 4.37.