In 1967, the Harvard sociologist Stanley Milgram conducted the experiment that led to what's become known as the 'six degrees of separation' theory.
But despite its popularity - it's even been used as the basis of a successful film - the theory's never been put to a proper scientific test - until now.
Yahoo and Facebook have teamed up for what they call the Small World Experiment, designed to test the hypothesis that anyone in the world can get a message to anyone else in just six stages, by passing it from friend to friend.
Milgram's original experiment was very small in scale. He sent around 300 letters to randomly selected people in Omaha, Nebraska, asking them to get the letter to a single 'target' individual - a stockbroker in Boston.
The senders were given some basic onformation about their target, and asked to send the letter to the person they believed was most likely to be closer to the target than they.
He found that those letters that reached their target did it in an average of six stages. However, only about 60 of the original 300 letters did actually succeed, making it a dubious theory to say the least.
"Unfortunately, absence of proof is not proof of absence, and until now there has been no way to resolve this argument one way or the other," says Yahoo.
"Because Facebook allows us to know not only the friends that people choose to send their messages to, but also the entire social graph of the 750M+ Facebook users, we can now test the hypothesis rigorously."
Yahoo is asking people to volunteer as either senders or targets, and will then ask senders to pass their messages via their Facebook friends. You can apply to take part here.
There are various rules - people must stick to using the Facebook friends they have already, for instance, rather than trying to friend people they think might be more useful.
What Yahoo doesn't explain is how it chooses its targets, beyond saying that a number will be recruited from volunteers. If it picks them at random, rather than selecting them to conform to world demographics, it won't tell us much about anything.
It might not seem that way, but most of the world doesn't actually have a Facebook account; Tuareg tribespeople aren't that likely to apply.