I think there's quite a few actually: http://isitrails.com (this site's database apparently has over 4500 Rails sites). For what it's worth, my site is built using Rails, and I'm building my software product using Rails.
I don't think there's much correlation between market size and the development platform. I'd say that Rails deployment is still harder than, say, PHP deployment and not as widely supported by hosting providers. On the other hand, I'm not sure there's anything like Engine Yard or Heroku for PHP.