If a page isn't LOADING AT ALL
If you're trying to get an embed code to work on https://developers.facebook.com/docs/plugins/page-plugin/ and nothing is appearing it's most likely due to the pages privacy settings.
Instruct your customer to do the following:
1. Click Settings at the top of your Page.
2. From General, click Country Restrictions, then click x next to any listed countries and click Save Changes.
3. Click Age Restrictions, select Public and click Save Changes.