Add Facebook Open Graph Tags To WordPress | http://www.paulund.co.uk/add-facebook-open-graph-tags-to-wordpress

functions.php

<?php

// Facebook Open Graph
add_action('wp_head', 'add_fb_open_graph_tags');
function add_fb_open_graph_tags() {
	if (is_single()) {
		global $post;
		if(get_the_post_thumbnail($post->ID, 'thumbnail')) {
			$thumbnail_id = get_post_thumbnail_id($post->ID);
			$thumbnail_object = get_post($thumbnail_id);
			$image = $thumbnail_object->guid;
		} else {	
			$image = ''; // Change this to the URL of the logo you want beside your links shown on Facebook
		}
		//$description = get_bloginfo('description');
		$description = my_excerpt( $post->post_content, $post->post_excerpt );
		$description = strip_tags($description);
		$description = str_replace("\"", "'", $description);
?>
<meta property="og:title" content="<?php the_title(); ?>" />
<meta property="og:type" content="article" />
<meta property="og:image" content="<?php echo $image; ?>" />
<meta property="og:url" content="<?php the_permalink(); ?>" />
<meta property="og:description" content="<?php echo $description ?>" />
<meta property="og:site_name" content="<?php echo get_bloginfo('name'); ?>" />

<?php 	}
}

function my_excerpt($text, $excerpt){
	
    if ($excerpt) return $excerpt;

    $text = strip_shortcodes( $text );

    $text = apply_filters('the_content', $text);
    $text = str_replace(']]>', ']]&gt;', $text);
    $text = strip_tags($text);
    $excerpt_length = apply_filters('excerpt_length', 55);
    $excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
    $words = preg_split("/[\n
	 ]+/", $text, $excerpt_length + 1, PREG_SPLIT_NO_EMPTY);
    if ( count($words) > $excerpt_length ) {
            array_pop($words);
            $text = implode(' ', $words);
            $text = $text . $excerpt_more;
    } else {
            $text = implode(' ', $words);
    }

    return apply_filters('wp_trim_excerpt', $text, $excerpt);
}