[dragonwriter]

> Advertisement is (to first approximation) zero-sum in the sense that what you sell, your competitor will not sell.

No, its not. A major portion of the effort in advertising of most products goes into spreading knowledge and/or perceived need for the product category, not just competing for share of the existing demand.

> The contribution to society of the advertisement is zero.

Even in the purely competitive case, that's only clearly the case in the case where the products are perfect subsitutes with neither cost nor utility differences for the purchaser.