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Those are actually very small numbers. Justin.TV's chat system, for comparison, has scaled to half a million concurrents. And I had to significantly diverge from the IRC spec to achieve that level of scale.

I'm interested in exactly how you diverged. What changes did you make to the protocol (especially in terms of server-server communication) to facilitate that kind of scalability?

The volume of communication is FAR higher than twitter though, and the number of engaged people is likewise higher.

It was also designed in the eighties to be used in university networks. The scale was a lot smaller back then.