This is what Facebook has to say:

“Site Maintenance” usually means that we’re making an improvement to the database that your account is stored on. While we’re doing this, you will not be able to access your account. In most instances, this maintenance will last only a few hours. Please note:

• After the maintenance is over, you will still be able to login with the same password.
• Your profile and account will not be affected by the maintenance.
• We don’t do site maintenance on all accounts at the same time, so it is possible that some of your friends will be able to access the site while your account is unavailable.
• It is not necessary to contact us about Site Maintenance. If your maintenance lasts for an extended period of time, however, please let us know.