In my 10 years or whatever of scanning ads, I think I must have seen exactly 3 that mentioned Linux and none that mentioned MySQL, PostgreSQL, gcc, Emacs or Apache. Admittedly I haven't been doing it
Well, ads always are not going to be the true indicator of market demand, especially those appearing in the print media.
There are specific sites available on the net that go on to show the market share of software tools. There are market analysts doing the job. You should check out for news from Gartner, IDC etc. For OS comparisons, you could check out www.netcraft.com.
Things aren't exactly the way you think it to be.
Raju