Always "real name" used, never username (API 2.0 hides usernames)

I have the same problem, for my project it's not acceptable to have a full name as a username. Plus, if someone who have the same name as an other person already registered, their username will look something like this : "John Doe.1"
So, I made a patch that gives you the option to use the facebook profile ID (the one in your "link") for the Drupal username.
I know it's not ideal but there is still the possibility to implement hook_username_access to hide this "ugly" username and replace it by another value (let's say data from custom fields attached to the profile)

This code will let drupal use the email address prefix as user name

function hook_user_insert(&$edit, &$account, $category = NULL) {

    // Strip off everything after the @ sign.
    $new_name = preg_replace('/@.*$/', '', $edit['mail']);

    // Ensure whatever name we have is unique.
    $new_name = helper_unique_username($new_name, $account->uid);

    // Replace with generated username.
    db_update('users')
        ->fields(array('name' => $new_name))
        ->condition('uid', $account->uid)
        ->execute();

    $edit['name'] = $new_name;
    $account->name = $new_name;


    return;
}

function helper_unique_username($name, $uid = 0) {
    // Strip illegal characters.
    $name = preg_replace('/[^\x{80}-\x{F7} a-zA-Z0-9@_.\'-]/', '', $name);

    // Strip leading and trailing spaces.
    $name = trim($name);

    // Convert any other series of spaces to a single underscore.
    $name = preg_replace('/ +/', '_', $name);

    // If there's nothing left use a default.
    $name = ('' === $name) ? t('user') : $name;

    // Truncate to reasonable size.
    $name = (drupal_strlen($name) > (USERNAME_MAX_LENGTH - 10)) ? drupal_substr($name, 0, USERNAME_MAX_LENGTH - 11) : $name;

    // Iterate until we find a unique name.
    $i = 0;
    do {
        $new_name = empty($i) ? $name : $name . '_' . $i;
        $found = db_query_range("SELECT uid from {users} WHERE uid <> :uid AND name = :name", 0, 1, array(':uid' => $uid, ':name' => $new_name))->fetchAssoc();
        $i++;
    } while (!empty($found));

    return $new_name;
}

dont forget to replace hook with module name

> So, I made a patch that gives you the option to use the facebook profile ID (the one in your "link") for the Drupal username.

This solution is probably about the only option available once Facebook removes the 1.x API next month. I think we should actually remove the username option entirely and provide an update hook to switch to real name (which is going to be better than Facebook ID in most cases).

I have used @villette patch and implement @quicksketch suggestion - "remove the username option entirely and provide an update hook to switch to real name"