Store SNS share value via function.php

Within a function or method, the return statement immediately stops the execution of the earlier.

Hence the last line never gets interpreted.

Switch the order of this:

return $social_score; 
update_post_meta($post_id, '_social_score', $social_score);

to this:

update_post_meta($post_id, '_social_score', $social_score);
return $social_score;

Update:

You are using three different variables for the post id, namely: $post_ID, $post_id and $post->ID. You should use the function argument: $post_ID.