That's exactly it. Before all of this interweb stuff the highest-margin ad venue was the phone book. Every business and trade professional spent at least 50 buc

That's exactly it. Before all of this interweb stuff the highest-margin ad venue was the phone book. Every business and trade professional spent at least 50 bucks and every lawyer, national chain, auto dealer, etc splashed out for more. YPPA are still pulling about $10B per year. The phone book is kind of the upper bound for behavioral and demo targeting: everyone who sees your ad wants to buy what you are selling right now. The only question is which ad they see first. A social network simply does not have that kind of audience. A site that is not in the business of telling people things they want to know is never going to make the same margins from ads.