This trend is readily evident in the IAB Internet spending reports for the last three years, where performance based advertising has gained share at the expense of impression based advertising models.
The biggest expense is advertising, which is significant at about 27% of revenues, but it is also very controllable and can be cut when times are tight to preserve profit margins.
The same report has revealed that Mayer's pushing to improve the consumer experience in its Homepage, Mail and Flickr offerings (amongst others) at the expense of advertising -- a move that'll win her plenty of fans used to the minimalist Google homepage.