From my experience, you can scale a rails app the same way you can scale a django app the same way you can scale a java webapp, and so on. The general problem with frameworks is they try to make you not think about datastore access. Datastore access tends to be the hardest part to scale.
Also, in addition to what others have mentioned, iLike uses (used?) rails - it was a large early facebook application. Not sure what their traffic is like nowadays, but I think it was pretty large in the early days. (Edit: http://blog.ilike.com/ seems to document their growth in registered users all the way back to 2007)
From my experience, you can scale a rails app the same way you can scale a django app the same way you can scale a java webapp, and so on. The general problem with frameworks is they try to make you not think about datastore access. Datastore access tends to be the hardest part to scale.
Also, in addition to what others have mentioned, iLike uses (used?) rails - it was a large early facebook application. Not sure what their traffic is like nowadays, but I think it was pretty large in the early days. (Edit: http://blog.ilike.com/ seems to document their growth in registered users all the way back to 2007)