Great post by Horace Dediu on how to look at Apple’s evolution over the next four decades:

My simple proposal is to think of Apple (and actually any company) as a customer creator. It creates and maintains customers. The more it creates, the more it prospers. The more customers it preserves the more it’s likely to persevere. This measure of performance for a company is not easy to obtain. It’s not a line item in any financial report.

The closest figure we have is that today Apple has one billion active devices in use. We’re not told of the total users or total customers because Apple cannot count people or wallets as accurately as it can count active devices. But as imperfect as it is, this number gives us a way to get close to counting customers.