Since the iPhone 6 went on sale last September, it has been a huge seller. Apple (AAPL -0.16%) reported that combined sales of the iPhone 6 and its larger sibling, the iPhone 6 Plus, topped 10 million during the first weekend of availability.

The iPhone 6 has sold in huge numbers since being released in September. Source: Apple.

Led by the iPhone 6, Apple went on to sell 74.5 million iPhones during the December quarter, and another 61.1 million in the March quarter. These unit sales numbers represented year-over-year growth of 46% and 40%, respectively, and are the top two iPhone sales quarters of all time by a wide margin.

Despite the massive demand for the iPhone 6 -- it's even outselling the iPhone 6 Plus by as much as a 3:1 margin -- it hasn't been the most popular iPhone as measured by usage share. That is, until now.

The iPhone 6 takes the lead
Mobile marketing firm Fiksu provides nearly real-time data on usage of various iPhone (and iPad) models, making it possible to track usage trends of different models over time. The data isn't perfect: Fiksu admits that its data for the Asia-Pacific region may not be representative, and China and Japan are both huge markets for Apple. But it still gives a pretty good sense of the adoption rate of various iProducts.

Since last July, the iPhone 5s had held the usage share crown among iPhone models. Source: Apple.

According to Fiksu's iPhone adoption tracker, the iPhone 6 finally matched the prior-generation iPhone 5s with 24.1% usage share last week. This week, it has pulled firmly into the lead with a usage share of about 24.7%, while the iPhone 5s has held steady at 24.1% of all iPhone usage.

Whereas the iPhone 6 has surpassed the iPhone 5s in terms of usage in the first week of May, the iPhone 5s didn't pull ahead of its predecessor -- the iPhone 5 -- until late July last year. This is further proof of the surging demand for Apple's iPhones.

The iPhone 6 Plus hasn't been quite as popular, but it still accounts for about 9.1% of all iPhone usage based on Fiksu's data. (Coincidentally, the iPhone 6 Plus also surpassed the iPhone 5c -- last year's second model -- in terms of usage share this week.) Combined, the iPhone 6 and iPhone 6 Plus represent more than a third of iPhone usage today.

Do the numbers add up?
At first glance, the Fiksu data showing that a third of iPhone users are already using the latest iPhones might seem at odds with Apple CEO Tim Cook's recent statement that "about 20% of the active installed base has upgraded to a 6 or 6 Plus." However, the two are not necessarily contradictory.

First of all, Cook appears to have been referring to the proportion of the existing iPhone user base that had upgraded. However, the iPhone 6 and iPhone 6 Plus have also attracted tens of millions of new users to the iPhone. This includes people switching from Android and other mobile platforms, as well as first-time smartphone buyers.

Second, people upgrading to the latest iPhones soon after they are released tend to be hardcore users. So, they may drive a disproportionate amount of usage -- in other words, the new phones could account for more than 33% of usage while representing a smaller portion of the installed base. (However, this factor may be offset by the popularity of the new models in Asia, where they are undersampled by Fiksu.)

Lastly, Cook's statement was an off-the-cuff answer to a question, so he may have been rounding. In short, while the Fiksu data is imperfect, it does seem to offer a reasonable approximation of adoption trends for Apple's latest iPhones.

Looking ahead
The past few weeks of Fiksu data indicate that the massive wave of demand for the iPhone 6 isn't subsiding yet. While usage growth is slower than it was during the first few months the iPhone 6 was on the market, it has held steady since mid-January. The combined iPhone 6 and iPhone 6 Plus usage share has been rising roughly 0.7 percentage points per week since then.

Apple's iPhone unit sales will still decline sequentially from the March quarter to the June quarter, but it will probably be a smaller drop than the 19% sequential decline recorded in the June quarter last year. This would lead to Apple reporting another quarter of greater than 40% year-over-year iPhone sales growth.

Based on these trends, it seems clear that the iPhone 6 will consolidate a dominant share of all iPhone usage between now and the eventual release of the next generation of iPhones this fall.