More data. Nielsen Online’s ad impressions per month, via Clickz.com (paid CPM display ads only*.)
First, the raw data.
* Per Nielsen: “Nielsen Online, AdRelevance service uses a proprietary methodology for estimating online advertising expenditures and only takes into account image-based technologies and advertising sold per CPM. Above data does not reflect house advertising activity, strategic partnerships between publishers and advertisers, or text units, paid search, sponsorships, email, units contained within applications (e.g., messengers and pre-rolls) or performance based advertising. “