In spite of the fact that consumers may not yet embrace the concept, "pop-unders" are subtly becoming ubiquitous. It is definitely an advertising medium that must be carefully selected since measurement so far is inconclusive on the reader's attitudes.
According to a new report from Jupiter Media Metrix, sites do benefit from pop-up or "pop-under" campaigns. (Pop-ups appear when you log onto a site; pop-unders appear when you close out.) These efforts boost page view numbers even though users themselves may have had little desire to visit the advertiser's actual Web site. In the case of one site, 73% of its 28.6 million unique visitors in June left the site within 20 seconds.
The pop-ups are Web pages that function as advertisements for that site. And every time pop-ups appear, they get counted as page views for the advertiser's Web site. Buyers beware regarding the actual viewership paid for!
Jupiter, however, feels it has an obligation to report any and all page view numbers, regardless of how those numbers are achieved. Doug McFarland, president of Media Metrix, said in the report "It is impossible to determine whether a user intended to visit a site that is also accessible through pop-up or pop-under pages. Truly objective audience measurement cannot be influenced by what a user may or may not have intended to do online."
You can read more here.