Everyone here has a pretty good idea of the score. The game was never designed to include trading, adding it would be a major design change. Why did they choose to do it this way? Not sure. It's probably not a matter of money, after all for the reasons stated above games with trading are generally much more profitable. I do know it was designed to be managed intimately, by a small group of GM's who pay a lot of attention to the game itself and its players. For reasons of balance and scams, a trading system would require a much larger staff to constantly police and regulate the system, investigate fraud claims and scan external sites for illegal auctions. Makes my head spin just thinking about it.
As for the other questions, yes, the average player only plays a few games a day, so 100 people on at a time is not bad. The Japanese game, when we started, I think topped out around 600 or 700. Considering the popularity there, the number of years they were up, and the number of accounts they had, I'd say we're on our way.
And friend referrals... we all want to do something like this. It's a bit difficult since we allow multiple accounts, though. As soon as we come up with a good way and work the logistics out, we'll put it before the Japanese.
"Scissors are overpowered. Rock is fine." -Paper