It might be undergoing some sort of repairing process, but normally it's just a temporary error in the Facebook's coding.
Try restarting your Internet Browser, or waiting a few minutes- then try again.
this is due to site maintenence..
It means that they are performing site maintenance. It does not last long. Expect to be back online within a few hours.
It means "one who uses and is a member of Facebook".
What it means is that this person had either deleted their account, has blocked you :( i hate it when it happens to me
it means to send a private message on facebook between you and the other person so not everybody sees it.