Advertisers have a cost-effective way to reach top-tier media sites: it's using ad networks that sell unused space on websites. When a site like the New York Times can't sell space, it turns to the ad networks to fill that space at a discount.
This process helps nytimes.com and the advertisers, too.
The catch is that the ad networks can't guarantee placement on the top-tier sites. But these ad networks also enable advertisers to target demographics that may not be possible through the Washington Post, for example.
There's no similar workaround for PR functions, but it's interesting to know these alternate distribution channels exist.
While they provide a service, these ad networks are also having a negative impact on newspaper sites. Check out this NY Times article, "Online Rally May Sidestep Newspapers."