Popular as in industry usage? Which is comparatively very small.
Smt88 is correct in that it is increasingly a niche. Churn rate is greater than retention rate. Junior Developers cant find jobs with Ruby and Rails, and they switch to others that do find them jobs. Companies only want to hire experience Ruby Rails developers but some of them moved on. It is a vicious cycle.
The point is even if Reddit was written in Arc, it has no bearing on Arc's industry status. It needs to be in a self sustainable ecosystem.
Ruby developers is small in % but rather large as an absolute number.