Query posts WITHOUT a custom meta field

You have to do a join or a subquery:

$querystr = "
SELECT $wpdb->posts.ID 
FROM $wpdb->posts
WHERE $wpdb->posts.post_type="post"
AND $wpdb->posts.post_date < NOW()
AND $wpdb->posts.ID NOT IN
( SELECT DISTINCT post_id FROM $wpdb->postmeta WHERE meta_key = '_social_' )
ORDER BY $wpdb->posts.score DESC
LIMIT 1;
";