This very point was covered by "Computeractive" Magazine (it's a UK computer magazine) in the issue of 19. March - 1. April 2009 (p43). This is what the magazine said:
(1) Got a Facebook account and want to get rid of it? For most people, the first step would be to click the Settings button on the Facebook menu bar and then click on the Deactivate link at the bottom of the settings list. However, this doesn't actually remove the account - instead, it puts it into cold storage, preserving all the settings and personal information in case anyone has a change of heart. Lots of people don't fancy the idea of having that info tucked away somewhere, just in case.
(2) To completely delete a Facebook account, use the browser's Back button to return to the main Facebook page and then click the Help link. When the page opens, type 'delete' into the search box at the top and then press Return. After a moment, Facebook will display a set of instructions that explain what to do and what happens to the account. Read these carefully and then proceed by clicking the 'here' link at the end of the paragraph.
(3) At the next screen, click the Submit button to continue and then at the next screen, type in the password associated with the account that's going to be deleted and then type in the required capture words. This is now the last chance to cancel. Still want to delete the Facebook account? Click the Okay button and after a moment the Permanently Delete Account dialogue box appears. Click Okay to finish up. Note that Facebook still retains the data for a further 14 days. Don't log on during that period and on day 15 the data gets zapped - forever.
That's what "Computeractive" Magazine said about deleting a Facebook account. I've never been on Facebook, so I haven't tried these instructions, but I have been told that they do actually work. "Computeractive" advice is very reliable. Best of luck.
"Computeractive" Magazine. 19. March - 01. April 2009.