Imagen destacada de WordPress como Facebook Thumbnail

functions-fb.php

/**
* Facebook Thumb Fixer
*/
add_action('wp_head','fbfixthumbnail');

function fbfixthumbnail(){
	global $post;

	//If there is a post image
	if (has_post_thumbnail()) {
		$featuredimg = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), "Full");
		$result = '
		<meta property="og:image" content="'.$featuredimg[0].'" />
		<meta itemprop="image" content="'.$featuredimg[0].'" />
		';
	}else{
		$result='
		<meta property="og:image" content="https://silicodevalley.com/wp-content/uploads/2016/09/facebook.png" />
		<meta itemprop="image" content="https://silicodevalley.com/wp-content/uploads/2016/09/facebook.png" />
		';
	}
	$result .= '
	<meta property="og:url" content="'.get_permalink().'" />
	<meta property="og:title" content="'.get_the_title($post->ID).'"/>
	<meta property="og:description" content="'.get_the_excerpt($post->ID).'"/>
	';
	echo $result;
}