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This usually means we're making an improvement to the database your account is stored on. While this process won't affect your account, you temporarily won't be able to access the site.
- Article on Facebook.com
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Keep in mind, we don't do site maintenance on all accounts at the same time. It's possible that your friends will be able to access the site while your account is still unavailable.
- Article on Facebook.com
However, if you're already logged into Facebook, you should still be able to use it if your account is affected, however, if you try to log out and back in again, you may be one that's affected and not be able to access your account.