A new report underscores just how long the long tail is in Apple's App Store. More than half of iPhone applications have less than 1,000 active users and only 5% had more than 100,000 users as of May, according to the latest monthly metrics report from mobile ad network AdMob.

The data is based on usage by 15.1 million unique users across the 2,309 free iPhone iPhone (and iPod touch) applications in the AdMob marketplace. The findings should come as little surprise to iPhone app developers already well aware of the difficulty of cracking the App Store's top 10, or even top 100 list.

A lucky few are also plucked out of obscurity to be featured on the store home page, giving them a huge advantage over competing apps.

With more than 50,000 free and paid apps now available, brands must build promotional expenses into the cost of launching new iPhone apps. Why do so many fail to gain more than 1,000 fans? "There are any number of reasons that an application could fall into this low-use category. The apps could be brand new, very old, targeted to a niche market, or not provide a good user experience," according to the AdMob report.

But the company argues that a significant portion (14%) of apps have built a strong base of tens of thousands of users. "As more and more iPhones and iPod touches are sold worldwide, usage of the medium and long tail of apps should increase," wrote Mike Fyall, AdMob's manager of product marketing, on the company blog.

For marketers advertising in third-party apps, how do they keep from getting stuck on apps that no one is using? Jason Spero, AdMob's vice president and managing director for North America, said advertisers typically buy certain audiences across mobile sites and apps in its network or placement on premium publisher sites rather than selected apps.

In its May report, AdMob also said that the iPhone accounted for a growing share of U.S. smartphone traffic on its network, increasing to 69% last month from 59% in April. It represents about half of global traffic. Smartphones now make up 37% of all mobile traffic in the U.S. as well globally.

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