AJAX, crawling, etc. How does facebook do it?

So I have been reading more on the #! “standard” google has created, and I recall Facebook using it heavily. However - I am not able to reproduce that anymore.

If I am in my newsfeed and click a photo, I see the photo pop up in a lightbox BUT THE URL does change to a /photo.php url. However, the page is not reloading… I still see rest of newsfeed in the background. Click the “Back” button closes this lightbox. How’d they do it?