Great musings on contextual advertising by John Battelle. He says that they aren't all they're cracked up to be because the advertiser has no control on where the ads will show up, and so they can have a real relationship with the audience, or the publisher, for that matter.
It's this relationship which I find entirely missing in all these contextual, behavioral, paid search networks. Sure, they are "relevant" to either a search, or to the content they match. But they are driven by metadata and the actions of only one of the parties – the content of the publisher for example (AdSense), or the actions of the audience (Claria, Revenue Science, Tacoda, etc.). As far as I know, none are driven by an understanding of the give-and-take that occurs between all three parties in a consensual relationship mediated by the publication. A site which has only AdSense or behavioral advertising fails to value (or monetize) the community connection between audience, publisher, and advertiser. Advertisers in these networks are not intentionally supporting the publication, and by extension they are not supporting the community the publication has created. In essence, they are not being good citizens of the community where their advertising is being displayed.
