Sort Main Posts by Post with newest Microblog Update

I have a function that sorts posts by last modified date: function custom_query_vars_filter($vars) { $vars[] = 'sortby'; //$vars[] .= 'another'; return $vars; } add_filter( 'query_vars', 'custom_query_vars_filter' ); function techslides_alter_query($query){ $sortby = get_query_var( 'sortby' ); if( $sortby=="modified" && !is_admin() && $query->is_main_query() ){ $query->set( 'orderby', 'modified' ); } } add_action('pre_get_posts','techslides_alter_query'); A ?sortbymodified gets added to the URL to return the sort order. How can I modify the sort order of the posts to be by which post has the newest microblog post OR update? The page I need help with: [ log in to see the link]