But, it doesn't matter how I feel about Google because Google isn't republishing content beyond what is allowed by fair use. So, even if I hated what they were doing, I don't have any legal justification for asking them to stop.
Individual comments make up a very small percentage of a page's content, so what we're doing is more similar to Google than you suggest (they also index your comments -- search for them). We display excerpts of pages; we attribute them liberally. A search result on Google leads you to the full page of content; a search result on BackType leads you to a full page of content. We index content for purposes of discovery i.e. search.
== Fair use
Also, it's pretty obvious that we're not in competition with content publishers, and we provide a very effective way to prohibit BackType from indexing comments.
I'm not talking about the rights of the weblog owners. I'm talking about the rights of the commenters on those weblogs. If you search for my comments on backtype, you get the full text of each comment. That goes far beyond fair use. You can search for any fragment of any of my comments on Google and it won't show you the whole comment, it only shows a (fair use) excerpt.
To be extra clear, you "excerpt" pages but you are not excerpting the individually-copyrighted comments. It is analogous to a website that republishes a book a page at a time and then claims that each page is a fair-use excerpt of the book.
Anyway, I don't see how you can say that your assertion is obvious when I've already explained the ways you compete with content publishers and content authors. Besides that, as the Plagiarism Today piece mentions, you compete in SERP placement since you let search engines index the content you plagiarize.