I've warned most of the people I've defriended that I was going to do so. Of course, 99% were former pupils of mine who kept on posting updates in gibberish (aka cellphone/MiXt language). I said if they did not stop, they'd be defriended. Then, the next time they did it, bye-bye. One girl wrote back in a snarky way, so I blocked her. As a teacher, I feel that one warning should be sufficient. And now she can write in gibberish all she wants, and I don't have to look at it.
I know exactly how many friends I have, and when I go to my profile, I quickly look at the number. If it's the same, no one has defriended me. I mean, what more do you need?