Twitter had scaling issues with Rails: http://engineering.twitter.com/2011/04/twitter-search-is-now-3x-faster_1656.html
Airbnb also outgrew Rails: http://nerds.airbnb.com/how-we-built-airbnb-holiday-cards-in-less-tha
Here's a couple links I found that addressed moving from a Monorail: http://david.heinemeierhansson.com/posts/33-myth-4-rails-is-a-monolith
http://www.slideshare.net/pcalcado/from-a-monolithic-ruby-on-rails-app-to-the-jvm
Does anybody here have any experience with migrating away from Rails? If so, please share any links and experiences.