Facebook said Monday the rapid development of the online social network had reduced the average number of people linking any two individuals in society to 4.74.

In the 1960s, American social psychologist Stanley Milgram and other researchers conducted several experiments to examine the average path length, or the number of steps along the shortest paths, for social networks of people in the United States. They suggested human society was a small world type network with a path length of around 5.5 or about six people on average.

In a new paper called "Four Degrees of Separation," researchers from Facebook and the University of Milan examined all 721 million active Facebook users with 69 billion friendships among them.

Using a set of algorithms developed at the university, the researchers found the average number of links from one arbitrarily selected Facebook user to another was 4.74.

"We found that six degrees actually overstates the number of links between typical pairs of users," said Facebook in a post on its site. It said the average distance in 2008 was 5.28, noting its development had brought the world closer.

However, some researchers said the random samples in the study were actually biased, noting that Facebook might have changed the definition of "friends."

Eric Horvits, a Microsoft researcher who led a similar study in 2008, told The New York Times his study used a more conservative definition of friend and found an average chain of 6.6 people among 240 million who exchanged chat messages. In the Facebook study, the notion of what a friend is has evolved.

Jon Kleinberg, a computer science professor at Cornell University, told the newspaper the social ties that make the world small are weak ones, on which people do not necessarily like, sympathize or have anything in common with each other.