[ladon86]

At ClassDojo we support many thousands of concurrent users on node.js using cluster to create multiple worker processes and using multiple boxes with Amazon ELB in front of them. All static assets are served from the CDN.

Handling your state in memory is fine for examples such as this, but in general you should defer all state to the database layer or use something like redis. That way your app server will remain entirely stateless so any node.js process on any box can serve a request identically - you can just scale up by adding additional boxes.