Dependency injection. How can I to load value from scopeConfig

[AlexaCMS]

Hi. How I can do this?

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Facebook\Facebook">
        <arguments>
            <argument name="config" xsi:type="array">
                <item name="app_id" xsi:type="string">socialsnet/facebook/app_id</item>
                <item name="app_secret" xsi:type="string">socialsnet/facebook/app_secret</item>
                <item name="default_graph_version" xsi:type="string">v2.5</item>
            </argument>
        </arguments>
    </type>
</config>

I try 

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Facebook\Facebook">
        <arguments>
            <argument name="config" xsi:type="array">
                <item name="app_id" xsi:type="config">socialsnet/facebook/app_id</item>
                <item name="app_secret" xsi:type="config">socialsnet/facebook/app_secret</item>
                <item name="default_graph_version" xsi:type="config">v2.5</item>
            </argument>
        </arguments>
    </type>
</config>

But it does not  work. Do you have some idea?