Rails, and the alternatives mentioned don't really make sense for a messaging application. You wouldn't write the back end for Gchat or AIM in Rails (or PHP, or Django), either. Rails would be fine as the web front end that talked to another messaging system. I've interviewed with the twitter guys and been to their talks and they definitely are NOT doing the right thing. However, "the right thing" is not easy and there are few people around and available who can do it.

edit: their best bet is to try to find someone who is burned out from working on wall street applications.