I have a few friends that clean out their facebook accounts from time to time. I have never done this but I think that it's a great idea. No use hanging on to the dead weight.
Here are some guidelines, if you will, for defriending:
1. If you see someone out in public and they don't acknowledge you, DEFRIEND.
2. On that note, if you see someone and you don't really want to acknowledge them, go ahead and delete 'em. Why string them along? It's not you, it's me. No, it really is you.
3. If their constant posts about their day to day, minute by minute "fiascos" are enough to make you want to put hot pokers in your eyes, then it's time to say "ADIOS".
4. If they have over 1000 personal friends then they won't give a crap about losing you, go ahead and bow out of that party gracefully. (And by gracefully I mean delete them, don't say a thing, and hope they never notice.)
5. If every profile pic consists of a self portrait with that horrid "kissy face", that would be enough for me to walk away.
6. If they "over like" or NEVER like a damn thing you say, post, share...buh-bye.
7. If you would never be friends or friendly with that person in "real life", why would you be friends with them in your "cyber life"?
8. If it becomes obvious that they are stalking you (ie. showing up where you are, consistently commenting on posts on your wall that are not meant for them, etc.), it is time to cautiously defriend. I say cautiously because you never know what those facebook creepers and stalkers are capable of. They might go all Single White Female on you.
So if you think that it might be time to clean house, hopefully these guidelines can help you out. Good luck.