Let’s say I want to disappear from Facebook without completely deleting the account (in case I want to keep my contacts, for example, or the content I have uploaded to the service). What steps should I follow to conceal my account as much as possible without having to remove it?
Go to http://www.facebook.com/deactivate.php. Your account will be suspended and not be displayed to others in any way, but you can always re-enable it by simply logging in again.
You will not lose any data, everything you entered as well as your list of friends and all application settings are preserved.
As stated in the FAQ:
To deactivate your account, navigate to the "Settings" tab on the Account Settings page. Deactivation will remove your profile and content associated with your account from Facebook. In addition, users will not be able to search for you or view any of your information.
If you’d like to come back to Facebook anytime after you’ve deactivated your account, you can reactivate your account by logging in with your email and password. Your profile will be restored in its entirety (friends, photos, interests, etc.). Remember that you will need to have access to the login email address on the account in order to reactivate it.