A real problem in both cases is preserving independence and integrity. Another problem Google and Robot Wisdom both face is that they are too well designed and consistent. Google users know where to find their search results, and that they are pretty reliable, so there's little incentive to look around and notice the ads. But I'd hate to see either Robot Wisdom or Google damage their functionality in order to improve the effectiveness of advertising. But I know of no easy solutions here.
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Robot Wisdom is just a lot cheaper.