Mobile phone advertising network AdMob revealed metrics for the month of May Friday that offer a detailed look at the pyramid-like distribution of actual app usage. In AdMob's network of 2,309 free, app-supported iPhone apps, only the top 5% (119) had 100,000 or more active users during May, while the majority, some 54% had fewer than 1,000 active users.

In the middle of the pack, 14% of the network's apps saw 10,000-100,000 active users, while 27% fell into the 1,000-10,000 range. All told, the firm said that there were 15.1 million unique users on iPhone and iPod touch devices in May 2009 in the AdMob Network.

Digging more deeply into the top 5% of apps, we see a microcosm of the broader trend, with approximately half of the top 119 apps getting approximately 100,000-250,000 or fewer active users. The graph curves up sharply towards the end with the top 10 apps in the network getting 1,000,000 or more users.

Chart courtesy of AdMob

Such distribution curves are common in many industries. For instance, broadband providers have long grappled with how to manage that 90% of their throughput comes from roughly just 10% of their user base. For iPhone apps, that distribution curve isn't so much a burden, but rather a goal of developers to get into the steeper end of the curve.

AdMob's data only represents less than 5% of the total iPhone app base of more than 50,000 apps, and doesn't include any paid apps at all. As a statistical sample, however, the firm's data is good benchmark for the broader free app market.