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

This technique is now deprecated.

This used to tell Google how to index the page.

https://developers.google.com/webmasters/ajax-crawling/

This technique has mostly been supplanted by the ability to use the JavaScript History API that was introduced alongside HTML5. For a URL like www.example.com/ajax.html#!key=value, Google will check the URL www.example.com/ajax.html?_escaped_fragment_=key=value to fetch a non-AJAX version of the contents.