The code change is below. Just so you know, in the 4.0 release (due out today/tomorrow), this code will be different. You'll still be able to do exactly what you're looking for, but the code to modify has been moved to a much better location. When you move to 4.0, just let us know, and we'll provide a similar (easy) code change.
To make the username automatically be the email address for Facebook User's Only in JFBC v3.4, edit the /plugins/authentication/jfbconnnectauth.php file. At line 149, you'll see the following.
$username = $this->_getAutoUsername($fbUser, $fbUserId);
$user['username'] = $username;Change that to:
// $username = $this->_getAutoUsername($fbUser, $fbUserId);
$user['username'] = $newEmail;The first line you're just commenting out so you don't run unnecessary code generating the 'new' username (that won't be used).
Let us know how that goes, and good luck!
Alex