Oh Dear poor facebook!Facebook appears to be unavailable for many users, our office included.
This looks like a DNS issue, looking up www.facebook.com reveals no answer, but facebook.com does resolve.
So I added this to my hosts file, its one of the addresses for facebook.com, and it all came rumbling back.
(update, I've updated the line to include the login hostname)
22.214.171.124 www.facebook.com login.facebook.com
Facebook.. if you're listening, fix your DNS, and you owe me!
 Windows users, use notepad, you'll find that file here:
Just copy'n'paste the line at the end