Mobile application downloads in the U.S. increased from 3.8 million in September 2011 to a record 4.91 million in October, a 29 percent month-over-month leap, according to mobile app user acquisition platform Fiksu's latest App Store Competitive Index.

Fiksu credits the download surge to the release of Apple's iPhone 4S, noting that the Index experienced download increases as high as 70 percent during the iPhone 4S launch weekend and 40 percent jump during the post-launch period compared to the weeks prior to the smartphone's retail launch. Fiksu also reports that organic downloads (defined as installs by consumers who actively seek and download apps without seeing an ad) grew 75 percent during the launch before settling in at 54 percent post-launch.

Fiksu adds that mobile marketing costs remained high in the days immediately following the iPhone 4S's debut, dropping in successive weeks due to decreased costs in traffic. By the end of October, the firm's Cost per Loyal User Index (which measures the cost of acquiring a loyal user for brands who proactively market their apps) netted out to $1.47, a 12 percent decrease compared to September's all-time high of $1.64.

"For mobile marketers seeking a window of extreme growth regardless of cost, the iPhone 4S launch presented a bonanza to secure large volumes of new users," said Fiksu CEO Micah Adler in a statement. "But for those seeking to maximize value and ROI, the best bet was to resist the temptation to join the initial rush. Our advice to cost conscious marketers was to wait it out for a week and then take advantage of the efficiencies in heightened volumes at significantly lower costs."