[Plugin: Facebook Opengraph] functionality question

I have checked a few such plugins and some have this code which your plugin doesn’t:

//Adding the Open Graph in the Language Attributes
function add_opengraph_doctype( $output ) {
		return $output . ' xmlns:og="http://opengraphprotocol.org/schema/" xmlns:fb="http://www.facebook.com/2008/fbml"';
	}

I am not sure if this is needed and even if I try and add it in, it doesn’t show in my sourcecode of the generated pages.
can you explaine what this is?

http://ww.wp.xz.cn/extend/plugins/facebook-opengraph/