change the $args to this:
$args = array(
'posts_per_page' => 20,
'paged' => $paged,
'post_type' => 'post',
'post_status' => 'publish',
'meta_query' => array(array('key'=>'favpost'.$userID, value=>'1', 'compare' => '='))
);
That is if the post_meta value is 1 when the user select that post as it favorite, or it could also be ‘true’ you will have to check that value on the custom fields area of the post edit.