There are two ways to interpret this: OPTION A) Omg, I'm not going to use Rails, even Twitter is ditching it. OPTION B) Hmm.. They built something really fast using Rails, got it out there, got massive popularity, made boatloads of $$ and now they're maturing and moving to a more robust infrastructure.
I go with Option B. If you're a new startup -- don't confuse this advice and start brewing your own Java-based server just because Twitter is doing this now.