Nothing complicated: Just paste the following code into your function.php file. Edit lines 4 and 5 according to your needs.

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;
}

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Thanks to TutsPlus for the cool tip!