Currently read several (popular science) books on ‘social network’ and ‘microbehavior’ (for two months, but haven’t really finished any — just only selected chapters of interest).
The idea is fairly simple. You have friends, and each friend of yours also has friends, and so on — by this way, the network will grows up in an exponential fashion.
The favorite claim is that, giving six degree of connectedness, we can link everyone in this world together into the same global network, i.e. Osama Bin Laden could be a friend of a friend of a friend of a friend of a friend of YOU.
And if we consider each person as a node and their friendship as an edge, giving enough degree of connectedness, we’ll come up with a graph (not tree) — that means you can know one individual person by more than one way (path).
Your ex-girlfriend is now a girlfriend of your best friend. — That’s also the case 😛