I wonder if there will be a service that hits the sweet spot of cool community instant messaging over a large group of people in the next generation of technology, I think so.
Taking down an IRC network (assuming DNS is not affected or users have cached list of servers in their client) is incredibly difficult.
Twitter doesn't fail-whale as often but it's still centralised and not overly reliable or robust in comparison.
Yes, but remember that DDoSing individual servers and taking over channels through the resulting netsplits was a favorite pastime of bored teenagers around the world.