Not all user activity is equal, and user activity is not constant. If your users suddenly become more active, for example because of a new feature they like, your load can increase significantly. User activity isn’t just a matter of the number of page views, either—the same number of page views can cause more work if part of the site that requires a lot of work to generate becomes more popular. Some users are much more active than others, too: they might have many more friends, messages, or photos than the average user.
Again...to buy time before implementing a scaling strategy.
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