Hm, I admit it's not so easy to explain the topic. Let me summarize it like this:
My site: WordPress
Issue: Regarding comments on a blog post
Comments: Usually WordPress uses it's own commenting system. Google can see the comments and this helps for SEO.
Comments on Facebook: To increase my comment rate, I am using a Facebook commenting plugin.
How does it work: Facebook uses an iFrame for comments, so they are visible in Front-End, but not in source code. But: The plugin is smart and takes those comments and puts them into the WordPress database. So WordPress shows the comments in the source code, but not in the Front-End. Only the Facebook comments are shown in the Front-End, but since it's an iFrame not in the source code.
So from Googles perspective the comments are visible in the source code, but they can't find them displayed in the front-end. The question is if that's a bad thing. The reason for me asking is, that some time back I read that lot's of black hats are using hidden content and that you should not do it.