When I was young, there was a popular saying to the effect that everyone knows someone who knows someone who knows the President. (US culture rather spills over the border) This is based on the idea of six degrees of separation. If you take any random 2 people in the world, you can find a chain of acquaintances between them that is at most 6 links long.
The idea originated in the 1960’s with the research of Stanley Milgram, a controversial social psychologist. He gave people letters with someones name on it they didn’t know and that lived in another city. By passing it only to people they knew, it took an average of 6 steps to get there. (Milgram was controversial due to his experiments illustrating peoples willingness to do apparent harm if they were not responsible)
In 2006, Milgrams research was reviewed and it was discovered that 95% of the letters sent did not reach their target. It was suggested the idea was simply an urban myth. But in that same year, Microsoft did an interesting study of the addresses of 30 billion instant messages sent in a given month. They found that any 2 people are linked by 7 or fewer acquaintances. They describe it as a possible “social connectivity constant for humanity”. The average is 6.6, with 78% linked by fewer than 7 links.
Of course, this is instant messaging users who are inherently more connected and more inclined to be. But it illustrates the point.
Anecdotally, we can see the validity. How often when you meet someone new do you discover common friends? Of course, this is more likely in sharing common interests. But if they are from another country? Services like LinkedIn, a professional networking service, show the number of connections in your network of contacts. When you search for a person in the network, the system shows you the degrees of separation you have from the person, if they are within 3. It’s easy to see I am 3 degrees from some of the famous like Bill Gates and Warren Buffet. Recently, I discovered a friend was surprisingly just 2 from Oprah when she needed a favour. You never know.