Online marketing firm, Hitwise, has published a socioeconomic demographic rundown of Yahoo and Google users. Check out this image.

According to Hitwise data and this Lifestyle Quadrant Analysis, while
lots of people are using Yahoo search (those are the dots in the upper
left), groups that have spent more that $500 or more online tend to use
Google (those are the dots in the lower right, the bigger dots
designate $500+ spenders). So while Yahoo has the "struggling
societies" market cornered, Google is fairly pleased with their
"affluent suburbia" and "upscale America" user base.

Via Hitwise