Check out this article for full details: http://news.cnet.com/8301-27080_3-20005556-245.html. Long story short they delete all of the data that can identify you when you delete your account. They do keep other data so they can track bugs but they are rather unclear about what that means.
Here is the important excerpt:
What happens to my data after I delete my account?
According to this Facebook Help Center page all personally identifiable information associated with your account will be purged from Facebook's database if you choose to permanently delete the account. "This includes information like your name, e-mail address, mailing address, and IM screen name," the site says. "Copies of some material (photos, notes, etc.) may remain in our servers for technical reasons, but this material is disassociated from any personal identifiers and completely inaccessible to other users. Facebook also does not use content associated with accounts that have been deactivated or deleted."
I asked a Facebook spokesman why copies would need to be kept at all and for how long, exactly when all traces of the data are gone entirely, and whether any data remain on any servers of partners, and if so for how long? Here is his reply:
When a photo or video is deleted, or
when a person deletes his or her
account, we quickly delete all of the
metadata for the photo as well as any
and all tagging and linking
information. For all practical
purposes, the photo no longer exists,
and we wouldn't be able find it if we
were asked or even compelled to do so.
This is similar to what happens when
you delete information from the hard
drive of your computer. Technically,
the bits that make up the photo
persist somewhere, but, again, the
photo is impossible to find. It's
possible that someone who previously
had access to a photo and saved the
direct URL from our content delivery
network partner (this is different
from the Facebook URL) could still
access the photo. However, again, the
person would have to know the URL, and
the photo only exists in the CDN's
(content delivery network's) cache for
a limited amount of time.