"I wish to know who's still using KDE, for example."
Well, I still use KDE. And I don't think I'm alone. You can get some feeling for the relative use of kde and gnome in Debian using popcon (http://popcon.debian.org). Of course it depends on the particular packages you choose, but comparing evince and okular, for instance, shows that kde is running at about 25% of gnome, which I don't think is too shabby for the non-standard desktop.