In earlier versions of Facebook, they were using hash (#) in their URL for AJAX calls to keep address bookmarkable which I can understand how to make it work since you can't change address part before hash (#)
But know on Facebook when you click on a photo, photo and comments opens in a black layer (theater mode), old content is still in the background and most important thing is it changes full address in the address bar.
Then you close black photo layer, it goes to previous content and address bar changes again to it's previous full address.
And all these happens without any hash (#) in the address bar.
Does anyone happen to know how this works?
Same thing happens on github.com too while browsing files and folders.