Twitter had issues with stability but I would say that they were under MUCH heavier than a 'non-trivial' load. I also think that it remains an open question as to whether the old twitter was just simply not well written at first or if it was really a fundamental issue with Rails. There are many very large websites that are claimed to be written in Rails. For example: github, airbnb, kickstarter, basecamp,...These seem to be scaling fine.
FWIW twitter switched out of rails before some of the huge performance benefits made their way into ruby itself. A rails stack could handle twitter capacity these days, given ruby/rails improvements made since.
