Add display mode.

module

/**
 * Implements hook_entity_info_alter().
 *
 * Add "Activity stream" view mode.
 */
function c4m_message_entity_info_alter(&$entity_info) {
  $entity_info['message']['view modes']['activity_stream'] = array(
    'label' => t('Activity stream'),
    'custom settings' => TRUE,
  );

  $entity_info['node']['view modes']['activity_stream'] = array(
    'label' => t('Activity stream'),
    'custom settings' => TRUE,
  );
}