Thanks to the high adoption rate of the iPhone 6 series, Apple's App store was very busy in October with approximately 7.8 million downloads each day, according to research firm Fiksu.

The download activity reflects a 42 percent leap from the previous month and a 39 percent increase year-over-year, according to Fiksu's App Store Competitive Index for October 2014. The index follows the top 200 free apps in the U.S.

"While recent indexes have shown steadily increasing costs, October presented an unexpected surge of another sort: volume," states Fiksu's report.

Fiksu cites the launch of Apple's latest smartphones as the prime factor behind such significant app download activity, actions the firm tracks in its Cost Per Loyal Customer Index. Because many consumers are re-downloading apps installed on previous devices, users are deepening their loyalties to the software and developers.

Fisku indicated ad dollars being pumped into mobile apps are expected to stay on their upward trajectory into 2015. For ad firms looking to maximize returns, Fiksu recommends tapping into Android apps.

The rise in the cost to engage users -- which Fiksu calls the Cost Per Install (CPI) -- is more pronounced on iOS than on Android. Based on Fisku's formulas, Android's CPL dropped by 26 percent, year-over-year, in October while iOS, in comparison, increased 24 percent.

"Combined with the slighter increases on Android's October CPI numbers, this may be a good place for marketers to spend advertising dollars as iOS costs only continue to rise," stated Fiksu.

While the historic App Store download in October further illustrates the high demand for the iPhone 6 series, another research firm is reporting the adoption of iOS 8 is lagging behind that of iOS 7.

Research firm AppLovin examined iOS 7 and 8's install base over the mobile OS' first 45 days on the market. While AppLovin states the numbers aren't absolute, it asserts its use of network activity is a solid metric for gauging the adoption rates of the two mobile OSes.

By day 45, iOS 8 had spread to just over 50 percent of Apple mobile devices while iO7 had already reached 80 percent in the same time frame following its launch. AppLoving states iOS 8's 5-GB size may be playing a role in hindering adoption of the mobile OS from catching up to iOS 7.

Along with gauging the adoption of iOS 8, AppLovin also found the 5.5-inch iPhone 6 Plus has been most successful in the Asia-Pacific countries.

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