I only did it once to unfriend people I'd had for Mafia Wars. I have somewhere around 350 friends. No, I don't know all those people, but I still wouldn't do a "sweep" of them. I'm also not trying to be "popular" or collect friends.
A very few I've communicated online with somehow (DIS, a parenting forum I used to frequent), but most of them are people from my town that I'd gone to school with. I'm involved with planning our class reunion, so I have most people from my graduating class to send out updates. I go to "all-class" reunions, so I have a lot of those people on my list. I see a lot of these people out and about in town and even though we'd never really talked in person, we have a good time joking around online and now that extends to real life.
There's one woman I'd never been more than passing acquaintance to since we were kids, and she has since become one of my dearest friends thanks to our interactions on FB.
Anyway, I guess that long-winded post was just to say, it is what you make it. If you want a smaller circle of close friends, then there's nothing wrong with that at all. But please don't diminish those of us who might have more that extends to just acquaintances and/or strangers. We all have our reasons. FB was a blessing to me after losing both my mother and one of my best friends a year and a half ago, and continues to provide me entertainment and friendship today.