Zuckerberg's company is testing an old theory:
Facebook researchers pored through the records of all 721 million active users, who collectively have designated 69 billion "friendships" among them. … To test the six degrees theory, the Facebook researchers systematically tested how many friend connections they needed to link any two users. Globally, they found a sharp peak at five hops, meaning that most pairs of Facebook users could be connected through four intermediate people also on Facebook (92 per cent).
The Dish 