As you may be aware, Facebook only provides a way to temporarily deactivate your account, not delete it. Less well-known is an innocuous little form buried deep within their site where you can submit a deletion request. Below I include the link, as well as a sample letter.
The Customer Support request form is not expressly for account deletion, but at least one individual has used it and reports having received confirmation from Facebook that the account was deleted.
Of course, your data would still reside in backups, and who knows if they secretly keep a copy of your data anyway? I recommend that you fill your profile with junk incrementally over a period of weeks before submitting the request.
As for the request itself, here's what I typed:
Please delete my Facebook account
I want Facebook to completely and irrevocably delete my account and all associated information. I understand that Facebook cannot delete messages I have sent to others (and perhaps comments I have left on journal entries) and that my info might remain in older backups, but all other info within or pertaining to my account should be destroyed.
If my information can also be removed from backups, I request that it be removed.
I hereby revoke all license, expressed or implied, for Facebook to store, display, use, or analyze my submitted information. This formally terminates my relationship with Facebook.
Their clickwrap EULA is non-negotiated, and so is my letter. If they can claim a license to my content, I can claim to revoke it. Sounds fair to me!
I'll follow up on this when they respond. (If they don't respond within a reasonable period of time, I'll start sending more requests, including by email.)
Update: I have received a response:
We have deleted your profile information and removed your email address from our login database. Please let me know if you have further questions or concerns.
Of note: The form I link to above apparently sends the message as an email to firstname.lastname@example.org, so sending to that email address is probably just as good a way of getting this done as using the form itself.