I'm unaware of any RoR that has successfully scaled,
Ruby is slower and more resource intensive than some alternatives, but clearly it's possible to scale to large numbers of users and developers with RoR:
That's not to say that choosing an alternative or rewriting a particular service or website in something simpler/faster/cheaper isn't sometimes a better option, but I don't think you can claim convincingly that Rails is impossible to scale.
Apparently Shopify has quite a bit of Go on the backend. I don't know any specifics about the rationale, so I can't tell if this means anything WRT Rails.
I'm sure they all have multiple services running on different tech. Rails is clearly not the one solution for everything, and any website growing in scale is going to hit hurdles and end up rewriting some of their logic, whatever the language/framework used.
Ruby is slower and more resource intensive than some alternatives, but clearly it's possible to scale to large numbers of users and developers with RoR:
http://www.groupon.com
https://www.shopify.com
http://www.yellowpages.com
That's not to say that choosing an alternative or rewriting a particular service or website in something simpler/faster/cheaper isn't sometimes a better option, but I don't think you can claim convincingly that Rails is impossible to scale.