Hi James,
To be honestly said, we would like to help you on this, but the facebook crawler is not handle by us.
Since you want to have the ability to redirect to /login when a user is not logged in from the page, at the same time when the Facebook want to crawl your steam content, they also do not have your site login access, so they can't retrieve any content from your site and come out display your homepage/require login content on the Facebook stream. Analogically, It was like there is a firewall blocking in betweem Facebook and your site, where the firewall was the login page
.
Thanks for your understanding James