Calling static method in the Widget Class

Try this instead:

$gloabl $myxclass;
$myxclass = new Xwidget();
add_action( 'xxx_followers', array(&$myxclass, "cron_addB" ) );

or

add_action('xxx_followers', "init_xclass_and_cron");
function init_xclass_and_cron() {
    $myxclass = new Xwidget();
    $myxclass->cron_addB();
}