In my experience, there don't seem to be many Japanese people on Stack Overflow, at least for questions about the programming language Ruby.
I'm aware of some users who are Japanese but have worked in English-speaking countries. I'm also aware of expats from English-speaking countries who are working in Japan using Stack Oveflow. But two Japanese programmers who live in Japan that I recruited to Stack Exchange haven't used Stack Overflow.
Is this because Stack Overflow is not well-known in Japan with Japanese people? Or are they aware of the site, but have chosen to not actively participate in it?