Ok, it's a little more complicated with FB. Sometimes they use AJAX, sometimes they reload the page (more often in Safari), but it looks liek it's AJAX, because side content and menu are same and are cached (loading really fast). You can check their URLs - if you don't see # sign and URL numbers are changing - the page is reloaded.

Why you DON'T want to have it working with AJAX: this would compromise URLs structure, SEO and user-experience. Imagine people sharing direct URLs to the media pages see more - that would be a mess. FB is a little different, because they're mostly "closed" network and can afford using adaptive methods. Look at Flickr for example - they keep all their URLs uniform.

Still, what we will do - we will add an anchor to the image position and will link with that anchor from "previous/next" arrow icons. That would always show pages scrolled to the beginning of the media files. Coupled with proper caching and "lean" media pages you would get same effect as in Facebook images browsing (perhaps we could also add the fancy/AJAXy "loading" indicator to ice the cake?).

http://www.boonex.com/trac/dolphin/ticket/2178