Platforms can tax externalities and generate costly signals

The word “URGENT” should cost at least $100 per usage.
— Merlin Mann (@hotdogsladies) May 24, 2013

One thing that’s great about platforms is that socially efficient, signal-generating Pigovian taxation like the kind proposed in this tweet is not a joke—you can actually do things like this, which may be one of the great advantages of markets mediated by a powerful third party.