hash sign in facebook urls

I wonder, why facebook.com (hypem.com, etc.) choose a hash sign after domain to path other files. like http://www.facebook.com/#!/reqs.php#event (#!/reqs.php).

What is the reason, and they map them w开发者_C百科ith javascript?

Google explains it here:
http://code.google.com/intl/en-US/web/ajaxcrawling/
http://code.google.com/intl/en-US/web/ajaxcrawling/docs/learn-more.html

Mostly the hash sign is used in urls so that when you visit it, you are taken to specific location in the page without your scrolling manually.

They do it like this so when, for example, you're in photos and you hit the photo Next button. This does an ajax request and gets the next photo. The problem with this is that the request doesn't change the url, and the user won't be able to Bookmark this new url!

So, they use the anchor part of the url to store these query string values.