Source: Apple.

With Apple's (NASDAQ:AAPL) iPhone 6 unveiling expected in just over a month, the Mac maker continues to grow its presence in the U.S. smartphone market. That's despite the typical seasonal trend in which potential iPhone customers delay purchases ahead of fall product launches.

Can I get your digits?
comScore yesterday released its latest estimates on the domestic smartphone market for the three months ending in June. The market researcher pegged Apple's market share of U.S. smartphone subscribers at 42.1%, up from 41.4% in March. That's well ahead of Samsung's 28.6% share, although the South Korean conglomerate put up the biggest sequential gain among OEMs from last quarter.

On the platform front, Google (NASDAQ:GOOG)(NASDAQ:GOOGL) Android remains the dominant platform in the U.S., with 51.9% of subscribers. That's a modest downtick from March, when the search giant grabbed 52.2% of the market. Both Microsoft (NASDAQ: MSFT) and BlackBerry (NYSE:BB) sit in the low single digits.

Smartphone platform

March 2014

June 2014













Source: comScore.

The iOS and Android duopoly isn't going anywhere. In fact, it's further solidifying.The Apple and Google operating systems combined owned nearly 94% of the U.S. smartphone market, up from 92% a year ago. Microsoft has been slowly growing its position, but mostly at BlackBerry's expense.

The bigger picture
There are now 173 million smartphone subscribers domestically, which translates into 72% penetration. Even with all the recent talk of smartphone saturation in developed markets such as the U.S., penetration has risen steadily. Mobile penetration was just 59% a year ago.

When it comes to market share figures that investors watch, there are typically two main flavors to keep a close eye on: unit sales and installed base. Market share estimates on unit sales from the likes of IDC or Gartner provide good insight into where the market may be heading, but installed base figures give a broader picture that encompasses more historical performance. Installed base estimates are less volatile in the short run, and it takes a lot more to move the needle.

For Apple specifically, installed base figures are extremely important because the company enjoys the highest customer loyalty in the industry. Looking at Apple's installed base gives a pretty good indication of how reliable its recurring revenue from iPhone upgrades will be. In contrast, Android OEMs typically have very little loyalty, even if those users are very loyal to the platform.

History repeats itself
Apple has steadily grown its installed base in the U.S., which is now an estimated 72.8 million iPhone users. That's a healthy uptick from the 68.7 million it had last quarter. That also shows Apple is successfully winning over new customers and first-time smartphone buyers in the U.S., who are among the most valuable buyers. Android now has 89.8 million domestic users.

Last year, Apple put up a pretty impressive gain with its installed base during the iPhone 5s/5c launch quarter. The company's installed base increased from 60 million in September to 65.2 million in December. While the preference for larger smartphones is much more prevalent in markets such as China, the two larger iPhone models that are expected next month will give Apple a better shot at converting Android users.

At the same time, existing iPhone users who have been longing for larger displays will all rush to upgrade. Apple still has plenty of domestic iPhone opportunities. It should be a "very busy fall" indeed.