I can't really speak for twitter but we use node.js happily at Soundkeep. It works greate for our use case of streaming audio processing. It has come a long way since it started. Obviously it isn't as mature at Ruby/RoR or Python/Django but not nearly as bloated either. The comparison is weak though since it isn't a framework but more of an environment. A lot of people like to think of it in comparison to the big frameworks though. Node.js is more about smaller libs and modules than any single framework.