How to order it ^_^ Just use mapReduce

order_by_likes_count.php

<?php
require_once "init.php";

$map = new MongoCode("function() {
  this.comments_count = this.comments.length;
  if (this.likes) this.likes_count = this.likes.length;
  else this.likes_count = 0;
  emit(this, {count: 1});
}");
$reduce = new MongoCode("function(k, vals) { return 1; }");

$sales = $wsCache->command(array(
    "mapreduce" => "wall",
    "map" => $map,
    "reduce" => $reduce,
    "out" => array("merge" => "wallCounts")));

$posts = $wsCache->selectCollection($sales['result'])->find()->sort(
  array('_id.likes_count' => -1, '_id.comments_count' => -1)
);

foreach ($posts as $post) {
  $post = $post['_id'];
  echo "{$post['_id']} has {$post['comments_count']} comment(s) and {$post['likes_count']} like(s).\n";
}

/*
 * OUTPUT:
 *
 * 4db8762fee60c6951b000000 has 0 comment(s) and 1 like(s).
 * 4db87648ee60c6951b000002 has 0 comment(s) and 1 like(s).
 * 4d984f2eee60c63e1d000031 has 3 comment(s) and 0 like(s).
 * 4db8774dee60c68318000006 has 3 comment(s) and 0 like(s).
 * 4db872ebee60c68318000002 has 2 comment(s) and 0 like(s).
 * 4d913020ee60c6843200000c has 1 comment(s) and 0 like(s).
 * 4db87475ee60c68318000003 has 1 comment(s) and 0 like(s).
 * 4d984f27ee60c6cd1b00002f has 0 comment(s) and 0 like(s).
 * 4db874e7ee60c6101b000001 has 0 comment(s) and 0 like(s).
 * 4db87511ee60c67919000003 has 0 comment(s) and 0 like(s).
 * 4db87639ee60c6951b000001 has 0 comment(s) and 0 like(s).
 * 4db8772bee60c6b21e000000 has 0 comment(s) and 0 like(s).
 * 4db8772dee60c63d0c00000a has 0 comment(s) and 0 like(s).
 * 4db8772fee60c6ae1e000001 has 0 comment(s) and 0 like(s).
 * 4db87748ee60c68318000004 has 0 comment(s) and 0 like(s).
 * 4db8774bee60c68318000005 has 0 comment(s) and 0 like(s).
 * 4db8774eee60c68318000007 has 0 comment(s) and 0 like(s).
 * 4db87750ee60c68318000008 has 0 comment(s) and 0 like(s).
 * 4db87752ee60c68318000009 has 0 comment(s) and 0 like(s).
 * 4db87754ee60c6831800000a has 0 comment(s) and 0 like(s).
 * 4db87756ee60c6831800000b has 0 comment(s) and 0 like(s).
 * 4db87758ee60c6831800000c has 0 comment(s) and 0 like(s).
 * 4db8775aee60c6831800000d has 0 comment(s) and 0 like(s).
 *
 */

$wsCache is a mongoDB object.

the best way:
create a property what count the specified entries ($inc) because you can create an index.