Nathan Myhrvold, in a conversation on Bill Gates's blog:
look at Wikipedia. For about 10,000 to 100,000 people it is an obsession that could properly be called entertainment. They edit the thing for fun – or anyway for zero monetary reward, so no economic stats there.
For the rest of the planet it is an amazing resource – incredible information on just about anything. Where does that fit in any economic analysis?
The whole conversation is worth reading, in part as a remedy to the complaint that, as Gates paraphrases it, "middle class salaries have not gone up much."