The Architecture Twitter Uses to Deal with 150M Active Users
I really question the current trend of creating big, complex, fragile architectures to "be able to scale".  These numbers are a great example of why, the entire thing could run on a single server, in a very straight forward setup.  When you are creating a cluster for scalability, and it has less CPU, RAM and IO than a single server, what are you gaining?  They are only doing 6k writes a second for crying out loud.