I encounter no ads that I am aware of, and it must cost money, which makes me wonder.
Stack Exchange is earning money by ads on the three largest sites (Stackoverflow, Superuser and Serverfault) as well as their Careers platform.
None of the SE 2.0 sites like Math earn any money at the moment.
please ignore this answer [see comments below]
I have seem ads here and actually of products that I found useful, unlike the rest of the web.