Facebook pages, profiles and communities are rank in the search engines like web pages. Specifically on Bing, but on Google and Yahoo as well. This is why Facebook pages ( business ones ) need to be handle in SEO terms, means content and incoming links. Think about it !
I agree about them being authority sites but in some ways (not all) doesn't that help them in terms of SEO to begin with?
All we know many webmaster make money just using social sites. Many business can start with social site.
Well, you have to remember - a lot of the social networking sites tend to have a ton of backlinks, so of course, Google and them will take note of them.