get_the_excerpt not working in header

I am using the code below to add FB Open Graph Meta Tags (trimmed to just show the description line. It all works apart from the description, which returns a blank string from the ‘get_the_excerpt()’.

//Lets add Open Graph Meta Info
function insert_fb_in_head() {
	global $post;
	if ( !is_singular()) //if it is not a post or a page
		return;
	echo '<meta property="og:description" content="' .get_the_excerpt(). '"/>';
}
add_action( 'wp_head', 'insert_fb_in_head', 5 );

Has anyone come across this before? Any help would be greatly appreciated.

Thanks

Jamie