Google has reams of company reviews, both those they've scraped and those they've solicited from the public. How hard could it be for them to downrank sites that advertise companies with relatively bad reviews, and uprank sites that advertise companies with relatively good reviews?

They could even scale the downranking so that the higher your site's reputation, the more it gets downranked if you're advertising poorly-reviewed companies. That would ding Forbes more than it would ding Joe's Little Blog, and prevent highly ranking sites (like Forbes) from having a monopoly on some search results.