Read the Terms of Service before you sign up
It's amazing how many people fail to read the Terms of Service. From http://www.facebook.com/terms.php:
When you post User Content to the Site, you authorize and direct us to make such copies thereof as we deem necessary in order to facilitate the posting and storage of the User Content on the Site. By posting User Content to any part of the Site, you automatically grant, and you represent and warrant that you have the right to grant, to the Company an irrevocable, perpetual, non-exclusive, transferable, fully paid, worldwide license (with the right to sublicense) to use, copy, publicly perform, publicly display, reformat, translate, excerpt (in whole or in part) and distribute such User Content for any purpose, commercial, advertising, or otherwise, on or in connection with the Site or the promotion thereof, to prepare derivative works of, or incorporate into other works, such User Content, and to grant and authorize sublicenses of the foregoing. You may remove your User Content from the Site at any time. If you choose to remove your User Content, the license granted above will automatically expire, however you acknowledge that the Company may retain archived copies of your User Content. Facebook does not assert any ownership over your User Content; rather, as between us and you, subject to the rights granted to us in these Terms, you retain full ownership of all of your User Content and any intellectual property rights or other proprietary rights associated with your User Content.
Although the Privacy help is somewhat contradictory:
If you're worried about who can see you and what they can see, remember that you have complete control over this and can edit your settings as you see fit from the Privacy page. If you still want to leave Facebook, you can deactivate your account from the "Settings" tab on the Account page.
Deactivation will completely remove your profile and all associated content on your account from Facebook. In addition, users will not be able to search for you or view any of your information. If you reactivate your account, your profile will be restored in its entirety (friends, photos, interests, etc.).
In either case, I think it's only appropriate that users take responsibility for their own data seriously. My dad always told me that unless you want the whole world to know something, never write it down. I think the same applies.
And, even if you had told Facebook to delete/deactivate your account, you still could have to contend with web crawlers, content indexers, and caches such as archive.org or google.com.