That’s the average path between users of Twitter. Pretty amazing.
From the link:
The ideas behind Stanley Milgram’s original “six degrees of separation” experiment, which suggested that any two people on earth could be connected by at most six hops from one acquaintance to the next, have been widely applied to online social networks.
On the MSN messenger network of 180 million users, for example, the median degree of separtaion is 6. On Twitter, Kwak et al. hypothesized that because only 22.1% of links are reciprocal (that is, I follow you, and you follow me as well) the number of degrees separating users would be longer. In fact, the average path length on Twitter is 4.12.
What’s more, because 94% of the users on Twitter are fewer than five degrees of separation from one another, it’s likely that the distance between any random Joe or Jane and say, Bill Gates, is even shorter on Twitter than in real life.