wordpress og:image featured image

Basically, I am using facebook markup on my website.

If an article has more than one image the facebook markup doesn't pick up the featured image for sharing, but does with only one image in the article.

I would like the facebook markup to pick up featured image no matter how many images are in the article?

Here is my markup:

<meta property="og:image" content="<?php if (function_exists('wp_get_attachment_thumb_url')) {echo wp_get_attachment_thumb_url(get_post_thumbnail_id($post->ID)); }?>" />

Does anyone know what needs changing?

Problem has been driving me nuts.

Regards