I've always been wary of the flaws in these studies. In general, you'd like to know what the overall campaign cost, at least. But in addition to that, being "exposed to the ads" to me still means you're also "exposed to the websites" and "exposed to certain communities" (or rather than being exposed to them, your likelihood of seeing these ads is driven by your membership and interests, which you had in the first place). The causal weight of the ads is still in question, if content and community are potential causes of sales patterns as well.
In this
SF Chronicle piece about Yahoo by reporter James Temple, we hear of a national retailer joining forces with Yahoo to measure sales lifts associated with a major display ad campaign. The good news: sales lifts came in at 5%, 93% of that coming in-store. The bad news: the sales lift figure for buyers under 40 was... zero.
If you're going to report the good, it's only fair to point out the bad. At least the ads didn't lead to a drop in sales.
