taptapAdMob, probably the largest server of ads to iPhone screens, has released its May Mobile Metrics Report, which you can download as a PDF file. The report breaks down the mobile ad market in several countries, showing which handsets are the most popular.

But the one set of stats in the report that everyone should memorize are those that measure the spread of popularity among apps. Here’s what AdMob found for the 2,300 apps that used AdMob to serve ads during May:

  • 5% (116) had more than 100,000 active users
  • 14% (322) had between 10,000 – 100,000
  • 27% (622) had 1,000 – 10,000
  • 54% (1,244) had less than 1,000 active users

Multiplying the number of apps times the number of active users shows that the top 5% of AdMob apps have at least 11.6 billion users among them. The bottom half of the market — the 54 percent under 1,000 each — have at most 1.2 billion active users.

So, AdMob’s top 5% of apps have at least 10 times as many users as the entire bottom half of the market. The gap is probably a few times that — somewhere between 10x and 100x.

This ratio shouldn’t be shocking. It’s typical in most media that a few blockbusters dominate the market, while a much larger number of also-rans are ignored by the crowd.

Digital media enthusiasts frequently invoke the concept of the  Long Tail, which holds that digital distribution makes it just as easy to serve 10 copies each of 1,000 unpopular apps as to serve 10,000 copies of a single hit. With all titles equally available, the theory says, consumers will fan out and buy obscure personal favorites more and mainstream blockbusters less.

AdMob’s report proves that isn’t happening at the App Store. Because people are a lot alike, and because most people like to mimic their friends’ behavior, iPhone owners are clustering around a small number of hit apps.  Now if only AdMob would tell us what they are.

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