A fellow student dug an article up - authored by DoubleClick - that basically says how online media helps with reach of a campaign. It was kinda self-serving, but raises a good point.

The internet must be addressed in any campaign. The article gave a good amount of information on reach/frequency statistics.

It's dated (3/03) but still relevant.

The key I guess is that just like in print, you need spend that has wide reach (horizontal)and strong specificss ones (vertical)