Big clients use a mix of long-term commitments and short-term spending, they advertise in several channels, they evaluate continuously (every minute or even mor

Big clients use a mix of long-term commitments and short-term spending, they advertise in several channels, they evaluate continuously (every minute or even more often), and they mix long-term purchases with short-term. If someone spends $10m/month in total and $2m/month on Twitter, that company might buy $1m/month in long-term discounted advertising and the rest at short-term prices (these are numbers I dreamed up). If Twitter's content or targeting becomes a little worse, Twitter loses the marginal advertising auctions, which can lead to zero short-term spending, because the long-term volume covers all that the client's code decides to spend there. (In other news, Facebook ad volumes increased recently.)