→ Klout
The social ranking platform, Klout, used to use a PHP & LAMP stack but found it difficult to continue to scale, so when they had the chance they moved to a Node.js backend. They even wrote about switching to Node.js and how they use it. [additional source: http://blog.klout.com/2011/10/the-tech-behind-klout-com/]
Very interesting post - thanks! Be sure to check out services like Cloudinary, Imgix and Firesize if you need such a scalable infrastructure and don't have the time code it yourself.
Very very cool, thank for sharing! Make sure to check out Cloudinary.com for image & video backend as a service, used by many photo sharing sites such as 500px.