Wrong variables in fb_stream_node_settings_form

You need to use $var instead $var_prefix in $form['fb_stream'][$var_prefix].

$var_prefix = 'fb_stream_enabled_';
$var = $var_prefix . '_' . $node_type;
$form['fb_stream'][$var_prefix] = array(
...

use this code

$var_prefix = 'fb_stream_enabled_';
$var = $var_prefix . '_' . $node_type;
$form['fb_stream'][$var] = array(
...

also in hook_node_presave you should to use this code

function MODULE_node_presave($node) {
  $fb_stream_enabled = variable_get('fb_stream_enabled__'. $node->type, 0);
  if ($fb_stream_enabled) {
  ...