I don't understand your correction, but my lack of understanding is really immaterial. I actually agree that twitter has a problem with economics if they cant sell anything. Whether it is worth solving the scalability problem is a good question. Certainly if they keep growing with no revenue, at some point it is definitely an economic problem. Indeed I posted a while ago about the community taking over twitter as a distributed public service (like email). I do think that there is an upper limit to the amount of messages the average user will send.