The number of friends differs widely. Some users have designated only a single friend, probably the person who persuaded them to join Facebook. Others have accumulated thousands. The median is about 100.
To test the six degrees theory, the Facebook researchers systematically tested how many friend connections they needed to link any two users.
Globally, they found a sharp peak at five hops, meaning that most pairs of Facebook users could be connected through four intermediate people also on Facebook (92 per cent).
Paths were even shorter within a single country, typically involving only three other people, even in large countries such as the US.
The Facebook team concludes, 'people are in fact only four worlds apart'.
Sociologist Stanley Milgram, who tested the six degrees theory in the 1960s, found an average of 5.2 intermediate people in the US.
At the time, he wrote that people at the end points were 'not five persons apart, but 'five circles of acquaintances' apart.' He thought of them as different 'worlds of acquaintances'.