Facebook Login apparently hanging forever in Safari

Hey guys

I've been working on this project, where the main crux of it is that Facebook users register as members of the site (with one-click registration enabled). As I've been testing, I've been unable to produce any errors.

However, as we're very very close to launching it, the clients have been bug-testing it, registering themselves on the site. For the most part it's fine, apart from Safari. Three separate people on different machines have all reported the same thing; when they click to register, Facebook's little 3-bars loading animation appears, and then nothing happens. When I check in the back end, all the people who reported a problem are not registered as users of the site, which suggests that it's Facebook that's never reporting back.

Being on a PC, I had to borrow a friend's Mac to test it, and it worked perfectly for me (typical), whether I was logging in or registering as a new user. I also tried it on Safari on my iPhone, and it also worked perfectly.

The only thing I noticed in a screenshot from one of the testers is that one of Facebook's JS files was perpetually pending in its response - specifically 5p0cp47osg-.js from ak-static.fbcdn.net - but I don't know how crucial this is to the login process.

Is this likely to just be a particular security setting that these people just have their browsers set to? They also have no problems in any other browser.