The latest, Forbes, announced Monday that it will start selling ads this spring for about 400 financial blogs. In recent months, Conde Nast, Viacom, CBS and other major media companies also have unveiled topic-specific ad networks to lure advertisers that want to buy more ads than any single site can sell.
If newspapers, magazines and broadcasters cannot expand online ad inventory, they are "under threat of becoming less and less relevant to the advertiser" said Russ Fradin, chief executive of Adify, whose technology runs ad networks for Forbes and others.
