My other favorite "scalability" study is from WhatsApp:

http://www.youtube.com/watch?v=c12cYAUTXXs

That's 'Billion' with a 'B': Scaling to the Next Level at WhatsApp

(that walk title was create before the acquisition and was mean to imply message count, after the acquisition it got a secondary meaning).

The one thing that is fascinating about it, is how small their team was compared to the volume and complexity of the operation.