The dutch government auth service (Digi-D) runs on it. So that's like 17MM people who have ID's there and login to do their taxes etc.
I don't know if they're Rails 3, but perhaps the New York Times or Groupon? Twitter certainly hasn't updated to Rails 3 yet.
Github either, AFAIK
Twitter uses Scala for most backend stuff...
NYT is mostly PHP with a few Rails microsites.
I believe most sites that reach really-really-big scale will have to start the technology carousel for optimization and etc... it's kinda hard to resolve this, because it seems to be a point of really massive scale while other's that are "very big" could be doing well