It is the very first time, I saw the following message when I enter facebook.com URL.
Your account is currently unavailable due to a site issue. We expect this to be resolved shortly. Please try again in a few minutes.
I google it and some answers come out.
One of it is:
For the official help on this error message please follow this link.
Many users have been receiving this error message for the past few days. You are probably receiving this message because facebook is working on the database that your account is stored on. Keep in mind that it is possible for you to receive this message and that your friend/spouse may not experience any issues (even if they are in the same household or city). If you have been getting this error message for more than 24 hours, facebook recommends clearing your cache and your cookies. Follow this link to find out how to clear your cache and your cookies.
I just wait less than one minutes, and refresh the page. Facebook is shown on my Firefox.
OK. I know, my facebook account is OK. Nothing wrong with it.