"Facebook Will Be Back Soon Facebook is down for required maintenance right now, but you should be able to get back on within a few minutes. In the meantime, read more about why you're seeing this message. Thanks for your patience as we improve the site."
The vast social network occasionally has to undertake various actions and institute fixes in order to keep it in working order. And perhaps Valentine's Day evening seemed like a time when people would be too busy eating at fancy restaurants and exchanging gifts to notice.
Also, Graph Search, a new Facebook tool that allows users to find friends and other things more easily on the site and with greater control than ever before, was just announced, so perhaps there are some bugs that Facebook has to work out with the new feature.
It's not clear exactly why Facebook is down at the moment, but Facebook provided the following stock answer:
"If you're seeing this message when you log in, it usually means that we're making an improvement to the database that your account is stored on. While we're doing this, you won't be able to access your account. We're sorry for the temporary inconvenience. After the maintenance is over, you'll still be able to log in with the same password. Your account will not be affected by the maintenance. We don't do site maintenance on all accounts at the same time, so it's possible that your friends will be able to access the site while your account is unavailable."
So don't worry, it is most likely that the work is not going to cause any trouble for you or your account.
However, if you ever find that you can't log into Facebook for 24 hours straight, the site's staff want you to contact them and let them know, which you can do by clicking on this link.