Load nodes with a fields condition
To filter by fields you should use fieldCondition() method instead of propertyCondition()

function get_list_of_users($site_id) { 
  $query = new EntityFieldQuery();
  $entities = $query-&gt;entityCondition('entity_type', 'node') 
    -&gt;propertyCondition('type', 'users_online') 
    -&gt;fieldCondition('field_website_id', $site_id) 
    -&gt;execute(); 
  if (!empty($entities['node'])) {
    $node = node_load(array_shift(array_keys($entities['node'])));
  }
}