Microsoft researchers studied the addresses of 30bn instant messages sent during a single month in 2006. Any two people on average are linked by seven or fewer acquaintances, they say.
The theory of six degrees of separation has long captured people's imagination - notably inspiring a popular 1993 film - but had recently seemed discredited.
One of the researchers on the Microsoft Messenger project, Eric Horvitz, said he had been shocked by the results.
"What we're seeing suggests there may be a social connectivity constant for humanity," he was quoted as saying by the Washington Post newspaper.
Read more at - 'Study revives six degrees theory'