add_action('wp_head', 'my_featured_fb_image');

function my_featured_fb_image($post_id = null, $size = 'full') {
if(tribe_is_event() && is_single() ) {$post_id = get_the_ID();
$image_src = wp_get_attachment_image_src(get_post_thumbnail_id( $post_id , $size ));
$fb_meta_tag = '';
if ( !empty( $image_src ) ) {
$fb_meta_tag .= '<meta property="og:image" content="' . $image_src[0] . '" >';
echo $fb_meta_tag;
}
else {
echo '<meta property="og:image" content="http://www.sciotomile.com/wp-content/uploads/2014/02/Default_Feature_Thumbnail.png"/>';
}
}}