What is #! seen in Facebook and Twitter URL?

Question

Possible Duplicate:
What's the shebang (#!) in Facebook and new Twitter URLs for?

I recently saw Facebook and Twitter's URL:

http://www.facebook.com/#!

http://twitter.com/#!/username

What is #! ? Anyone?

Answer 1

It has to do with Google's method of making AJAX applications "crawlable." See here.

Answer 2

The # is the hash sign, used for referencing an anchor in HTML. Javascript can read the text after a hash in a URL, making it usable for AJAX solutions. Changing the anchor will not navigate away from the page, but it will update the browser history, allowing you to navigate back and forth. As for the !, it likely is just used internally, it doesn't mean anything in a URL.