Perhaps you already know if Google does not index the content inside an iframe, so the comments from Disqus or Facebook is not indexed.
But now Google has started indexing the content inside an iframe, I realized this today when I checked the HTMLy blog demo, Danlogs.
The blogs using Disqus comments, and it turns out all the comments on the blog indexed by Google. A certainly good news for Disqus users.
For Facebook comments, I have not yet figured out its already indexed or not, but it should also being indexed because it uses an iframe as well.