In July 1999, there was a news article stating that PC usage had doubled yet again, reaching penetration of 50% of homes. People were hailing this as yet another sign of the inevitable climb of technology, but I saw it as a warning sign that the glass was half empty, and coined the following:
will never double again
(in any number of months).
To be clear, it all depends on what you count. If you're counting units sold, you can double your count by selling everyone 1 unit, then 2, then 4, etc. (In Finland I understand that cell phone usage is above 1 per capita, but still growing.) If you're counting the total number of households that own the product, you can double your count by doubling the population, or by convincing everyone to divorce and become two households. But if you're counting percentage of people (or households), there's just no more doubling after you pass 50%.
So the next time you hear about the doubling of another technology, remember you heard it here first.