Brands are quickly realizing that in order to build sustainable app businesses, it’s going to take much more than just reaching sky-high download numbers. It’s becoming increasingly clear to developers, publishers and marketers alike that forging real relationships with users through ongoing engagement to keep them coming back for more, is critical. But in an industry where there is no simple definition of a "loyal user," and no standard metric exists for measuring ROI beyond app downloads, measuring campaign effectiveness and optimizing spend remains an elusive challenge.

As the mobile app landscape continues to move ahead at lightning speed, our team has remained focused on meeting this velocity head on – while helping brands remove the guesswork from the complex and ever-shifting mobile marketing equation. After spending several months closely analyzing almost two billion mobile app actions captured by our platform, we were thrilled to launch our inaugural Fiksu Indexes last week to help marketers benchmark their performance against industry averages while empowering them to make more intelligent business decisions.

We believe that loyal users are the key to sustained growth for mobile app business. Our Cost per Loyal User Index not only tracks the fluctuating cost of acquiring loyal users – defined as a person who opens an app three or more times – but also provides a standard measurement that is applicable across all app categories. Our App Store Competitive Index analyzes the average aggregate volume of the daily downloads of the Top 200 ranked free iPhone apps in the U.S. to illustrate how specific ranking positions become more and less competitive over time.

By focusing on app marketers’ two biggest challenges - cost and volume - we’ve seen an overwhelming response to our Indexes in these first few days. Here are a few industry reactions:

GigaOM’s Ryan Kim has long believed that app developers and marketers need to move away from the simple pursuit of pure downloads and search instead for engaged and loyal users in order to remain successful. He shares his take on our Indexes here - What’s a loyal mobile app user & how much do they cost? He writes, "I think [Fiksu] raises an interesting point and is right to try and get at this question. For developers, the real challenge is to acquire users that come back. These are the people that can be monetized through advertisements and will buy in-app purchases. These are the people that can keep a game like Tap Zoo at the head of the top grossing charts list for the better part of a year."

In examining the Fiksu Indexes, ReadWriteMobile’s Sarah Perez writes, "Both tools could certainly function as worthy complements to whatever trend-tracking, analytical resources developers, publishers and brands already have at their disposal."

Kim Mai-Cutler of Inside Mobile Apps sat down with Micah Adler, Fiksu’s founder and CEO, for a comprehensive Q&A on the Index findings, as well as a deep-dive into how user acquisition has changed since Apple’s recent ban on incentivized downloads. You can check out their full conversation here.

For more information, charts and our full analysis of the Index data, visit our Index resource library. Stay tuned for more as the Indexes will be published monthly. And for all you Android users out there, check back soon since we will be expanding our Indexes to include tracking and analysis of the Android market in the near future.