Well the survey would include Contiki if engineers chose to mention it.
This survey indicates that less than 2 percent +/- about 2 percent of engineers are using Contiki.
But as you indicate, engineer usage is not the same as installation base.
OOPS, sorry, I switched colors so your comment is right. I wonder however if those figures (all, no only for uC/OS) refer to effective use or are only "intend to use sometime in the future". I remember a few years ago this issue was raised for this or another similar research.