Snippets → WordPress

add_filter('user_contactmethods', 'my_user_contactmethods');

function my_user_contactmethods($user_contactmethods){

  $user_contactmethods['twitter'] = 'Twitter Username';
  $user_contactmethods['facebook'] = 'Facebook Username';

  return $user_contactmethods;
}

Source: http://wp.tutsplus.com/tutorials/quick-tip-add-extra-contact...