It's a small world online too: an unusual U.S. experiment has found that an average of only six e-mail messages is needed to randomly contact anyone else on the Internet.
The study in the current issue of the journal, Science, lends weight to a famous study 35 years ago that we are all somehow connected to each other by a chain of about six social relationships - known as 'six degrees of separation'.
Its findings could help scientists to understand how social networks develop, how diseases spread or even to track down criminals, the authors said.
Dr Peter Dodds and colleagues at Columbia University in New York, asked volunteers to help relay a message to a target person, by forwarding the message to a social acquaintance chosen on the basis that they were perceived to be 'closer' to the target than themselves.
More than half of all participants lived in North America and were middle-class, professional, college-educated and Christian, "reflecting commonly held notions of the Internet-using population", the team noted.
These volunteers were randomly allocated one of 18 targets from 13 countries: the targets included a U.S. university professor, an archival inspector in Estonia, a technology consultant in India, a policeman in Australia and a veterinarian in the Norwegian army.
Senders were also asked why they considered their nominated acquaintance a suitable recipient, which helped to determine the way in which the searches progressed.
The researchers then tracked the resultant communication, which involved 61,168 individuals from 166 countries and 24,163 distinct message chains. The messages that reached their targets took around six forwards to get to their destination.
"When passing messages, senders typically used friendships in preference to business or family ties; however, almost half of these friendships were formed through either work or school affiliations," Dodds said.
"Successful chains - in comparison with incomplete chains - disproportionately involved professional ties rather than friendship and familial relationships. Successful chains were also more likely to entail links that originated through work or higher education," the authors wrote.
Men passed messages more frequently to other men, and women to other women. This tendency to pass messages to a same-sex contacts was strengthened by about 3% if the target was the same gender as the sender and similarly weakened in the opposite case, the researchers found.
More than half of the choices participants made when passing on a message were based on the next link in the chain living closer than themselves to the target, or being someone who shared the target's occupation.
The researchers say their results run counter to previous suggestions that certain highly social individuals with a wide range of personal contacts tend to act as 'hubs' for the exchange of messages.
"Participants relatively rarely nominated an acquaintance primarily because he or she had many friends, and individuals in successful chains were far less likely than those in incomplete chains to send messages to hubs," the report said.
The original 'six degrees of separation' experiments were performed 35 years ago by the influential American psychologist Stanley Milgram, who concluded that any individual is connected to any other through a short chain of social ties, the average chain length being six people.
"Milgram named the 'small world problem' after the obligatory cocktail party response of strangers who unexpectedly discover that they share an acquaintance," said Mark Granovetter of the department of sociology at Stanford University in California, in an accompanying commentary.
"Thus, the very name of the phenomenon alludes to surprise at and ignorance of one's social network," Granovetter said. "Indeed, Milgram liked to recount that before his results became well known, he would ask generally sophisticated audiences to guess how many personal acquaintance links were required to connect randomly chosen endpoints in the U.S. Many guessed in the hundreds; hardly any imagined the half dozen or so of his and later experiments.
"This excellent new study raises - but cannot resolve - the important question of how much people know about their own social networks and why this matters," said Granovetter.