var result = db.game5.aggregate(
  {
    $match: {
      ca: {
        $gt: ISODate('2013-02-14T14:06:03Z'),
      },
      s: {
        $gte: 30,
      },
      'us.0': {
        $exists: true,
      },
    },
  },
  {
    $unwind: '$us',
  },
  {
    $match: {
      us: {
        $ne: '',
      },
    },
  },
  {
    $group: {
      _id: '$us',
      number: {
        $sum: 1,
      },
    },
  },
  {
    $sort: {
      number: -1,
    },
  },
  {
    $limit: 10,
  }
);

printjson(result);