Twitter probably couldn't scale because of bad engineering, not because of RoR. For example, Shopify embraces the Ruby ecosystem and it scales up very well.
I imagine shopify is more profitable for the amount of traffic it gets, so they can afford to throw more machines at the problem as well.
Shopify does probably have better engineering though. They have an amazing process of 2 code reviewers per PR, and one code reviewer is from another team. They write tests for everything and deploy continuously to production.
