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Palo Alto-based Focus Ventures has invested $15 million in San Mateo-based ad network Turn, which plans to use the money to increase its New York sales presence and provide what AdWeek calls "a cushion should the Internet economy take a downturn." Lots of people think such an downturn is underway — online ad spending is definitely hurting — and lots of people think ad networks are at the root of the problem. Not Turn CEO Jim Barnett, natch.Ad networks, he told AdWeek, are "a very effective way for publishers to sell a lot of their inventory. They can get $1.50 from their network partners or they can get zero." If it were so simple, Focus's investment would make more sense. A recent Interactive Advertising Bureau and Bain & Co. study of seven publishers showed that the publishers in the group who used ad networks heavily could only sell their premium ad invetonry at an average CPM, or cost-per-thousand impressions, of $10.36. The ones which mostly abstained were able to charge CPMs over $20.