Well, i think i've narrowed it down to the NMS host's firewall settings, as they try to se what the problem is they obviously discover that when clicking the FB connect it throws up an outgoing IP address, so, they add that to the firewall and bingo it works, however, cause FB literally have many IP addresses that get used by the entire universe single users get problems and the site crashes.
When they turn the firewall off completely everything works perfectly, but, of course they are not gonna and nor do i want them to disable the firewall altogether.

So, i wanted to contact FB to see if they would kindly advise their IP range (which i strongly doubt) but, as you say it looks like you can't actually contact FB, which, is utter pish, they are happy to take zillions of USD but interact directly with their community. Something that eBay had to rectify.

Anyway, the current position is, the FB link is inactive until the host can figure a way to allow their firewall to work with that link.

vBulletin have bent over backwards in helping me, am even on direct PM with one of their leading techs, but, it's in the hands of the NMS host.

Strange thing is, my bike site works perfectly well