A good analysis from Cory Doctorow:

Imagine how creepy it would be to wander into a co-worker’s cubicle and
discover the wall covered with tiny photos of everyone in the office,
ranked by “friend” and “foe,” with the top eight friends elevated to a
small shrine decorated with Post-It roses and hearts. And yet, there’s
an undeniable attraction to corralling all your friends and friendly
acquaintances, charting them and their relationship to you. Maybe it’s
evolutionary, some quirk of the neocortex dating from our evolution into social animals who gained advantage by
dividing up the work of survival but acquired the tricky job of
watching all the other monkeys so as to be sure that everyone was
pulling their weight and not napping in the treetops instead of
watching for predators, emerging only to eat the fruit the rest of us
have foraged.