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In my case it was a foreign website from Tokelau (.tk extension) that was taking up a good deal of my ad impressions.
YPN's quality and performance needs to be improved in this regard as this can hurt both YPN and publisher earnings. YPN needs to:
1) Screen domain name extensions and filter them out at the advertiser level (as long as the network is in non-international mode).
2) Periodically ping and test the landing page to determine if it contains real content. Those that fail should be flagged and the advertiser notified and/or kicked out of the network.