EDIT:
ok, i just fixed it. always use plugins_url() in plugins instead of the wp constant WP_PLUGIN_URL. i changed this in wp-social-login.php and it’s working fine… 🙂
(background: WP_PLUGIN_URL keeps the unmapped url, plugins_url() returns the mapped one we need here.)
to get the dynamically generated slug of a custom post type (archive) you can use:
$obj = get_post_type_object('post_type');
$slug = $obj->rewrite['slug'];
