If the ad exchanges aren't exchanges, what are they?

I should note that I don't claim to have had an original thought in my post yesterday on why ad exchanges are not like financial markets/exchanges*. In addition to Huayin Wang's insight in the comments, @jonathanmendez points me to a Cogblog post touching on this, and Jordan Mitchell of Rubicon mentions his own post from several months ago.

I love this stuff, excellent reading.

Keep in mind, though, that exchanges are only one type of market. Most commodities are not traded on exchanges. Most things are not bought and sold on exchanges. So, while I defend talking about the stock market as an analogy and as a way to think about what ancillary companies can exist in the ecosystem and how important they will be, maybe it would be useful to specify what the marketplace for ads is, not just what it is not. I'll try to blog on that next week.

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* My ex-trader friend, mentioned yesterday, scoffed at my use of the word "exchange" in our conversation, saying "if it's an exchange, then the CFTC would like to talk to you."