eMarketer: Why Advertise Online?
This article discusses the issue in the context of recent research on affluence. For my money the top factors are:
The internet's ability to reach an at work/school audience
Use of the internet to source product information
Lot of people are presently trying to make the case for internet advertising. This is often done without even a passing mention of relative cost. All advertising works to some degree, to judge how viable a particular medium is you have to consider effectiveness in the context of price. For instance, in 1999 internet advertising may well have been the least cost effective form of advertising ever. Now with improved techniques, more sophisticated measurement criteria, and much lower prices, that situation has certainly changed.