NO, it's not!
If we quickly check githut.info, we can see number of repositories, commits and pushes to github currently.
Ruby is still the 6th most used language on github repositories. Although with emerging of front-end frameworks and libraries like AngularJS, ReactJS and many others there can be seen a decline in usage of Ruby on Rails.
Picking up again
With Rails 5, active cable and other new technologies integrated in Ruby on Rails, it can pick up on popularity in the future. Still, there is large community that still commits to gems which made Rails popular in the first place.