siteoptimo

<?php
// put this in your functions.php
add_filter('pre_user_query', function(&$query) {
   if($query->query_vars["orderby"] == 'rand') {
       $query->query_orderby = 'ORDER by RAND()';
   }
});

// Query will look like this:
$args = array('orderby' => 'rand', 'number' => 5);