Okay, this is the bit I don't understand, so it's probably not worth your effort to try to explain it to me. Real friends last a very long time. Acquaintances come and go. But either way, I'm me. I don't have separate, carefully cultivated personae. If there's someone I haven't seen for a while, but want to get in touch with, there is almost always going to be a better, easier way to do it: usually, mutual friends. I can see why someone else might choose Facebook as a way of doing it, but when Frances and I were trying to think of anyone we might try to use Facebook to trace, we couldn't think of any. If they're gone, they're gone. Otherwise, we can find them if we care enough.
Facebook, to me, loooks like a giant, year-round, 'round robin': you know, those awful letters that some people send out at Christmas to everyone they know, full of news that is of little or no interest to anyone, generally boasting of their and their family's achievements. Most of my friends regard these as being in appalling taste, and I suppose that for many of us, the same feeling carries over to Facebook.
It also strikes me that Facebook can be a excellent way of pretending you have friends, without really making any effort to maintain the contact on a personal level: "I care enough about you to tell you this personally, and I think you are a good enough friend that the news may be of interest to you." Otherwise, you can broadcast your life (and Facebook is pretty much broadcast) and let your 'friends' pick up the pieces that suit them. To me, that's not friendship. At best, it's an illusion of friendship.
You probably won't agree with this viewpoint, but at least you may understand (in the Californian phrase) "where I'm coming from".
Cheers,
R.