Do you think any system goes from 1 to a billion users without being re-written? It never happens, if you go if into that architecture astronaut mindset from the beginning you will not ship the features necessary to get traction. Saying that Twitter faced language-level issues is absolute nonsense. Any stateless web front-end scales trivially whether it's Rails or anything else, it's the data flow and caching that needs to be designed properly to solve Twitter-type scaling issues.