Today Empire Avenue rolled out the Twitter Index. Previously users could only buy shares in other Empire Avenue users. But now any Twitter person can be bought and sold… and it has spurred activity on Empire Avenue! I’ve talked in the past about how much I like the buy-and-sell gameplay system of Empire Avenue and am delighted to see it extended to users outside of the Avenue. Basically an emphasis has been put on making Empire Avenue’s strongest feature (the buy and sell) even stronger by removing some artificial roadblocks to network growth and expanding the user base. With the Twitter Index, users can expect even more growth, and hopefully…