It's "solved" for advertisers who can measure ROI, at least short-term. It's unsolved when the success metric for the campaign is not easily measured, like bran

It's "solved" for advertisers who can measure ROI, at least short-term. It's unsolved when the success metric for the campaign is not easily measured, like brand awareness. It's also not solved for the publishers not engaging in arbitrage or fraud who earn less and less per impression. Let it play out unchecked and the bad actors will kill the market by driving all the legitimate suppliers out of business: if ad rates are driven so low that content sites can't pay their staff with the ad dollars, they'll either go out of business or exit the market (i.e. erect paywalls, return to direct ad sales instead of participating in exchanges, etc).