because your front page is a static page, you might need to add something to the code to fill a default og description in this case;
example (first section only shown – keep the rest as it is):
<?php if(is_single() || is_page()) { ?>
<meta property="og:type" content="article" />
<meta property="og:title" content="<?php single_post_title(''); ?>" />
<?php if( is_front_page() ) { ?>
<meta property="og:description" content="<?php bloginfo('description'); ?>" />
<?php } ?>
<meta property="og:url" content="<?php the_permalink(); ?>"/>
<meta property="og:image" content="<?php if ( $thumb[0] == null ) { echo $default_img; } else { echo $thumb; } ?>" />
or even replace
<?php bloginfo('description'); ?>
with a static text.