Since you asked this publicly, I assume you're looking for a public answer. Else I'm sure you'd have PM'd me instead.
I've only deleted a handful of Facebook friends since I started using Facebook (maybe around 10 people total). This always happens for one of two reasons:
- Spamming (sending solicitation messages or posting spam on my wall). You ever spam me about anything on Facebook, I drop you as a friend immediately.
- Exhibiting a pattern of posting lame or immature comments on my wall. If you do things that annoy my other Facebook friends, causing people to start complaining to me about it, I drop you right quick. Just don't have time for that sort of thing.
There's a hard limit of 5,000 Facebook friends, and I'm already past 3,800, so I'll probably hit that limit later this year.