I deleted my Facebook account 6 months ago for pretty much the same reasons as the other posters here. The site just wasn't useful to me anymore. It brought only negative things into my mind. Just lots of comparing my life to other lives - mostly people I haven't talked to in 3, 5, 8 years. I realized I really didn't care about what those people were doing, and that what they choose to post on FB probably isn't an accurate representation anyway.
One of the big things that kept me on there was the thought that I might some day want to talk to some of these people again. The fact is, I didn't talk to anyone on there. The people that ARE really important all have my phone number, or email, or know where I live, and can contact me one of those ways. Being on Facebook wasn't actually helping me make or maintain any connections; It just kept old connections I don't have anymore, hanging around in my mind and on my computer screen. People will come and go in life, and Facebook was making it difficult to let them go.
Besides this there are all the privacy implications of Facebook which is another reason I deleted it. I actually tried to make a new account under a false name at one point, so that I could still use it to contact people without people being able to look up my name. I found that I was unable to activate the new account without providing a phone number - undoubtedly to cross-check that with existing accounts to see if it's a duplicate. Since I only have one phone number, I stopped right there.
As it stands my account has been deleted for half a year, and the last time I actually used FB was almost a whole year ago. The only regret I've had is that I forgot to save some of my pictures. I have had no reason to open a new account.