Emarketer has a good overview of current research supporting online advertising:
- The audience is more affluent
- It’s effective in changing users’s perceptions of brands.
Robert Losch at Marketing Fix adds
- It’s the best way to reach the work/school audience
- Consumers are using it to find product information
I would add that it’s the best or even the only way to reach people who have begun to limit their consumption of commercial media: TV nonviewers, DVR users, or NPR listeners. Many of the people are as influential as they are difficult to reach.