As you can see in the chart below, the market follows a predictive logistic function (thin blue line). Notably, the iPhone closely follows the same pattern as the market (red line).
Dediu suggests that we can forecast the iPhone's growth by extending the line to the point where the U.S. market reaches saturation.
We also know from the plot of the market that F = .91 is reached around February 2017. So we can suggest that at 90% penetration (approximately saturation) the iPhone will have 68% market share of users in the US. Forecasting the addressable market (US population aged older than 13) at about 266 million that implies 180 million US users of the iPhone by early 2017.
Check out the chart below for more details...