Facebook Share Image Fix for WordPress Theme

<?php
function tnc_share_image_fix(){
     if(has_post_thumbnail()){
        $thumb_url = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), array( 600,315 ), false, '' );
        echo '<meta property="og:image" content="'.$thumb_url[0].'" />';
    } else {
        echo '<meta property="og:image" content="'.get_template_directory_uri().'/images/placeholder.png" />';
    }
}
add_action('wp_head', 'tnc_share_image_fix');
?>