Airbnb, Kickstarter, Twitch, GoodReads, Bloomberg, there is quite a list of companies who still run a significant amount of their site through Rails. Over time I imagine that particular bottlenecks and performance critical pieces have been pulled out into other languages and services as necessary, but I certainly wouldn't feel bad about it if I was developing or maintaing a Rails site today. It's significantly better than PHP, and a hell of a lot of sites are still run on that. The reality is that if you really need to scale, your primary bottleneck will probably be the datastore, not the frontend.