Facebook
Plug-ins

Infuriatingly, although this did work for me initially, it mysteriously stopped working and at this point in time I have not manage to work out why or get it to work again

To add a Facebook 'Comments Plugin' you need to generate the appropriate code at https://developers.facebook.com/docs/plugins/like-button 

The code comes in two parts:

The first part comes with the instruction: Include the JavaScript SDK on your page once, ideally right after the opening body tag
The second part comes with the instruction: Place this code wherever you want the plugin to appear on your page. 

To add to Google Sites you just need to combine these two code snippets together and embed using the Embed Code tool