Custom code for flag activity

/**
 * Implementation of hook_flag(). Trigger actions if any are available.
 */
function hbi_flag($action, $flag, $content_id, $account) {

  // Follow another user
  if ($action == 'flag' && $flag->content_type == 'user' && $flag->name == 'follow') {

    $message_id = 'heartbeat_follow_user';
    $variables = array(
      '@user1' => l($account->name, 'user/' . $account->uid),
      '@user2' => l(user_load($content_id)->name, 'user/' . $content_id),
    );
    heartbeat_api_log($message_id, $account->uid, $content_id, 0, 0, $variables);
  }

  // Become a fan of another user
  if ($action == 'flag' && $flag->content_type == 'user' && $flag->name == 'fan') {

    $message_id = 'heartbeat_flag_userfan_add';
    $variables = array(
      '@user1' => l($account->name, 'user/' . $account->uid),
      '@user2' => l(user_load($content_id)->name, 'user/' . $content_id),
    );
    heartbeat_api_log($message_id, $account->uid, $content_id, 0, 0, $variables);
  }

}