They may still use Ruby On Rails on the front end, and for some internal systems, but these days, there's a lot more involved. A significant part of the system is a queue - written in a language called Scala, based on the same technology as Java.
The developer of the new Scala based queue wrote about it's development on his journal.
The big mistake Twitter made with their original code, wasn't any language choice, but an architecture choice. Initially it shared more in common with a CMS, or blog. You enter a piece of text, and it's fetched back on demand. The work they have done to put a queueing system behind it has done far more towards its stability and scalability than anything they did to the front-end code. You can search for more on 'twitter scaling' to read more of what they have done in this style, and what others have thought about it as well.
