The very first line in your js kills the function
var user = document.getElementById('username2').value;
You have no username2 ID. But you have a username ID. Maybe a misprint?
BUT Biggest question is "Why come back from ajax_info.php to submit again to the server? Why not have ajax_info.php call fbauthorize.php? Or just include the code of fbauthorize.php into ajax_info.php and make the alert(xmlhttp.responseText) say Mission accomplished data stored.'"?