I noticed a few days ago that my Facebook XMPP chat account from Pidgin had stopped working, popping up a weird error message: “Not authorised”. This happened even though I did not change any of the account’s settings.
The resolution is pretty Voodoo or, how Vlad described it, the error needs a Microsoft-type fix. Apparently if you are logged in on Facebook using your email address instead of the user name, the XMPP protocol might not authenticate you. Therefore, if you’d like to use Facebook’s chat on third-party clients while you’re still logged on in a browser, make sure you always log in using your user name instead of your email address on Facebook’s site. Trying to use your email address in Pidgin for the user name field results in an error, as email addresses are not valid XMPP user names.