First and foremost, sorry for the bump - but this is an interesting addition to this discovery of the way Facebook changes pages.
That code is from IE6ify, a funny little joke for designers that if you put that code into a bookmark on your bookmark toolbar and keep clicking it, it randomly screws up your page.
Now why is this relevant?
I was on this thread a couple days ago, wondering exactly how Facebook DOES change its pages. And after sifting through the very caddy immature responses, and hallow threats to report to a MOD (for no reason) ... I discovered there was a misconception. On one end, people were claiming that it is impossible, that the bottom bar refreshes, as does the whole page. On the other end, (the right end), people were saying no it doesn't, it uses an iframe to bring in the content, etc, notice the hash in the address bar.
With the IE6ify script I included above, you can CLEARLY SEE it does NOT refresh the page. In fact, even in Safari it does not refresh it. YES, I know, it does in Chrome & IE, but those are irrelevant to this case because it does NOT in Firefox and Safari, making the question remain, how DO they do it? I think hjb provided a very good amount of information toward this, and want to comment quickly on that.
First let me preface what I'm about to say with IDGAF about my 'rep' or 'my post being removed' with this... but I think it's absolute BULL that there is SO MUCH immaturity on forums these days. We should all be here to help each other out. That's it. I absolutely HATE when people state "facts" and shut out any other suggestion from anyone else. Its absolutely ridiculous.
With that said, happy coding!