Om Malik writes about the rise of online ad networks:
that there are some other problems which these networks will have to contend with: the availability of crack sales teams with deep relationships with the advertising community, that typically moves at a pace slower than the Silicon Valley. The shortage of advertising sales talent is the Achilles heel of the new Web 2.0 ecosystem. (Ironically, the old media has a built in advantage here.)
Think about it this way desktop applications that were once sold as shrink-wrap is now advertising supported. 411 services that cost as much as $1.99 a minute are being replaced by ad-supported services, and well even AOL is kissing a multi-billion dollar access business goodbye and replacing it with an ad-only model. Even Bill Gates has espoused a future where many of Microsofts services will be bolstered by advertisements.