I perma-deleted my account several weeks ago. I was fed up with watching the not-so gradual erosion of privacy which has been occurring since I first opened my account in 2006.
My problem is that the contract, whether legal or implied, that I had with Facebook offered me a certain level of privacy, and protection of data. This is what Facebook built their reputation on. Because of this contract, I uploaded a lot of personal photos, notes and information and generally became deeply involved in the service.
Every time Facebook have a new API release or re-design, however, I became familiar with the uneasy awareness that another slice of this personal information was about to become available to businesses, friends of friends, the general public and goodness knows who else. The final straw for me was the realisation that my friends list was going to be made public, and there was nothing I could do about it. There is no way I want the list of my personal friends and acquaintances being made public: it is just beyond the pail. In hindsight, I should never have put that information on the web at all, but there you have it -- I trusted Facebook to look after that information, and Facebook repeatedly broke my trust, and that's why I've left.
I believe that there is very good reason to protect the basic tenets of our privacy online, and Facebook have shown themselves singularly incapable of doing that. As well as that, I am highly unimpressed with the direction and quality of the product, particularly the UI which has devolved from one of the best on the web to a cluttered, unpleasant mess.
I haven't missed Facebook once. Nowadays, I stick to Twitter, where the contract is clear. Everything is public, and we all know where we stand.