[User23]

It's also hard to understand because it's largely illusory. If, say, Facebook is down and ad spending ceases for an hour, that money didn't just go up in smoke. It's still in somebody's ad budget, and there's a very good chance they're still going to spend it on ads. Thus, while there will be a temporary slow down in ad spend rate, over the course of the quarter the average may be completely unaffected due to catch up spending to use the budget.