Through Mashable, found a link to an excellent article at Adweek that talks about why the value of a pageview at web 1.0 companies like Yahoo! are higher than the value of pageviews at social network companies like Myspace and Facebook.
The short of it is that audience engagement matters as well as pure volume of pageviews. If users are not spending much time on pages, they are not in the mood to click ads. This is compounded for performance based advertisers as multiple explosure to the same audience eventually leads to dramatically lower click through rates. But don’t let this summary dissuade you from reading the whole thing.