How do I detect a Facebook signup is complete in new window

How do I detect a Facebook signup is complete in new window

I have a page where I want to open a window to various social signups -
one of them being Facebook. In the new window, when the signup is done, it
currently redirects to a page that uses postMessage on the window.opener
As I found out in my last question ( How do I detect window opener in IE
), this doesn't work in IE because it's cross-domain.
Meanwhile on a site like Soundcloud, the popup opens, completes, closes,
and then redirects the opening page. I have no idea how they can be
pulling that off given the cross-domain restrictions.
Is there a way to preserve values across redirects in a window?