What's the shebang/hashbang (#!) in Facebook and new Twitter URLs for?

_x000D_ _x000D_ I've just noticed that the long, convoluted Facebook URLs that we're used to now look like this: http://www.facebook.com/example.profile#!/pages/Another-Page/123456789012345 As far as I can recall, earlier this year it was just a normal URL-fragment-like string (starting with #), without the exclamation mark. But now it's a shebang or hashbang (#!), which I've previously only seen in shell scripts and Perl scripts. The new Twitter URLs now also feature the #! symbols. A Twitter profile URL, for example, now looks like this: http://twitter.com/#!/BoltClock Does #! now play some special role in URLs, like for a certain Ajax framework or something since the new Facebook and Twitter interfaces are now largely Ajaxified? Would using this in my URLs benefit my Web application in any way?