We use the users's private key of their Ethereum wallet to sign every ad impression . Combine that with our KYC requirement, and now advertisers know that every ad impression was a real person.
 - https://github.com/clearcoin/extension/blob/dev/app/scripts/...
We're trying to grow our own ad economy however. That's why our Ad Bidder platform  only accepts ClearCoins (XCLR) as payment for placing your ad.
 - https://platform.clearcoin.co