On the one hand the idea of a reverse search engine is somewhat appealing, on the other hand; it's Clippy for the internet.
I see you're trying to deny global warming. Would you like to:
1. research the available facts and science?
2. Have an authority figure you trust tell you, you're wrong?
3. Meet other like-minded singles?
On a more serious note; a marketing service that put it's customers brands in front of people who didn't yet know that those products were what they were looking for would be an awesome startup from a business perspective and a somewhat creepy one from a human perspective.
e.g. You're talking about medical condition X, therefore I shall try to sell you drug Y even if you don't mention Y or drugs at all in your search terms/blog post/etc. And I don't care if you even knew that drugs for X existed.