There is an interesting article in Business Week about the changing landscape of the internet advertising marketplace.  One quote:

Web users have learned to shop around more, and now, instead of clicking on ads from the little guys and buying from them on the spot, they're often buying from the big brands they know well. If many of the small and midsize companies that pioneered this ad medium get disillusioned, search ads could lose their luster.

Businesses who advertise via the internet will adjust as consumers change their behavior.  Consequently, advertising providers will also need to adjust their offerings in order to continue to grow.