Having watched the rise and fall of SixDegrees, Friendster, and the many other proto-hominids that make up the evolutionary chain leading to Facebook, MySpace, et al, I'm inclined to think that these systems are subject to a Brook's-law parallel: "Adding more users to a social network increases the probability that it will put you in an awkward social circumstance." Perhaps we can call this "boyd's Law" for danah boyd, the social scientist who has studied many of these networks from the inside as a keen-eyed net-anthropologist and who has described the many ways in which social software does violence to sociability in a series of sharp papers.
Here's one of boyd's examples, a true story: a young woman, an elementary school teacher, joins Friendster after some of her Burning Man buddies send her an invite. All is well until her students sign up and notice that all the friends in her profile are sunburnt, drug-addled techno-pagans whose own profiles are adorned with digital photos of their painted genitals flapping over the Playa. The teacher inveigles her friends to clean up their profiles, and all is well again until her boss, the school principal, signs up to the service and demands to be added to her friends list. The fact that she doesn't like her boss doesn't really matter: in the social world of Friendster and its progeny, it's perfectly valid to demand to be "friended" in an explicit fashion that most of us left behind in the fourth grade. Now that her boss is on her friends list, our teacher-friend's buddies naturally assume that she is one of the tribe and begin to send her lascivious Friendster-grams, inviting her to all sorts of dirty funtimes.
In the real world, we don't articulate our social networks. 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.
Keeping track of our social relationships is a serious piece of work that runs a heavy cognitive load. It's natural to seek out some neural prosthesis for assistance in this chore. My fiancee once proposed a "social scheduling" application that would watch your phone and email and IM to figure out who your pals were and give you a little alert if too much time passed without your reaching out to say hello and keep the coals of your relationship aglow. By the time you've reached your forties, chances are you're out-of-touch with more friends than you're in-touch with: Old summer-camp chums, high-school mates, ex-spouses and their families, former co-workers, college roomies, dot-com veterans... Getting all those people back into your life is a full-time job and then some.
You'd think that Facebook would be the perfect tool for handling all this. It's not. For every long-lost chum who reaches out to me on Facebook, there's a guy who beat me up on a weekly basis through the whole seventh grade but now wants to be my buddy; or the crazy person who was fun in college but is now kind of sad; or the creepy ex-co-worker who I'd cross the street to avoid but who now wants to know, "Am I your friend?" yes or no, this instant, please.