Beyond Dot-net from January 2009

That's not "exactly" true. The stock market is a zero-sum game. The market capitalization is the total amount of money that investors/speculators have placed into the "money pot" that we call the stock market. When investors get jitters, they sell their shares and take their money out of the money pot. No money is created or destroyed in the stock market. So when the market drops by $1.2T, it means that investors withdrew that much from the stock market (but not necessarily their brokerage accounts).