What I think Facebook want if possible is the address (the bit that comes up in the address bar usually at the top of the Explorer window - ie for this thread http://mudcat.org/thread.cfm?threadid=121472&messages=854#2678762 ) of the offending message or identity.

It is probable that falsely assuming the identity of another to assist the BNP is an actionable libel under English law - but you have to get hold of the identity of the identity thief to sue them.

You may be able to do this under US "safe harbour" doctrine, which, as I understand it, provides for a common carrier to be exempt from libel liability so long as it identifies the poster of the libel. It's probably easier to do this under those provisions than the UK law as it stands after the "Motley Fool" libel cases. Heric may have more info on the US law here.

If the identity thieves are using material in which you own the copyright you may be able to requir Facebook to take down the infringment by using a DIgital Millennium Copyright Act notice (US law, so again Heric maybe able to add information).

I have listed some of the impersonators of which I know on the "Will the Real Richard Bridge Please Stand up" thread.