the uk and canadian laws are much more similar than to the american laws, and you can sue facebook and the john doe libeller in the uk and collect against assets in the uk. in fact don would just need to sue in the uk and forgo damages against fb, just get the identity information and seek damamges and an injunction against the uk libeller. assuming he is in the uk. and i would be surprised if there was an americaqn cyber stalking don in the uk. although wierder things have happened.fb has some uk assests to collect against and would likely cooperate once they had a court order against them. they would avoid damages by cooperating.

if it turns out to be libel from a far well then there are some criminal avenues to be tried and reciprocial enforcement of judgements treraty. however fb would police the guy much more.