Why exclamation is used in URLs? [duplicate]

Answer 1

It's because it is an AJAX crawlable notation for URLS, an effort driven by Google to define a convention allowing Web 2.0 websites to be indexed by search engines.

And of course check the perfectly valid answer for What's the Shebang in the new Facebook and Twitter URLs?

Answer 2

It's a convention used for AJAX requests, one that lets google crawl that ajax content. There are some other major examples of this, twitter for example.

In Chrome (and other browsers as they support it) you won't see those URLs, since facebook is also using some HTML5 history features to change the URL in the address bar/browser history.