This very question was recently covered in "Computeractive" Magazine (19.03-01.04.2009), so here is what the magazine said: (1) go to main Facebook page and 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. (2) 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" had to say about removing a Facebook account. If you just click the Deactivate link at the bottom of the Settings, your personal information is simply put into cold storage. It doesn't disappear from Facebook's files. Best of luck.