You're thinking too qualitatively; imagine the spikes they get... Also most apps are I/O restricted, not CPU...

Also, all these proposed messaging architectures are somewhat flawed. Twitter isn't really "sending" messages to other people. It's more like a person's message history is bound to their account, and appropriate privileges are applied. Then, when people try to "read," privileges are obeyed and information is produced...

Am I the only one who believes the solution to Twitter does not involve a massive distributed system? Twitter is inherently centralized... people don't see that.