Behold, marketers — some iPhone numbers you can work with, finally

It seems like a report comes out every week with new statistics on the Apple App Store phenomenon. And the story is almost always the same: The App Store is huge and getting bigger.

Today, comScore released the newest such report breaking down the cumulative base of applications installed via iTunes, thereby revealing the true size of the audience an individual app can reach. This actual audience size has been a major point of interest for marketers and developers for a long time. After all, the number could be used to estimate how much revenue an app might bring in with advertising or premium features. In the report, comScore also exposes the U.S. penetration rates for the App Store’s top 25 applications based on their number of installs. Here’s what it found:

The 32 percent penetration rate for Tap Tap Revenge means that the app has been downloaded by nearly one out of every three iTunes users. What else does the chart show? Twelve out of the 25 most popular mobile apps were games, including classics like Hangman and Pac-man. That’s pretty much what a typical VentureBeat reader would expect. In a survey conducted for our recent GamesBeat conference, we found that Apple’s iPhone will be the gaming industry’s favorite platform going forward. Among non-gaming applications, Stylem Media’s “Backgrounds” applications had the highest installation base, followed closely by the top social networking applications, Facebook and MySpace Mobile.

But it’s the penetration data that will probably spark the most debate within the mobile industry. Two weeks ago, mobile advertising network Admob came up with a pretty good estimate for the size of Apple’s mobile user base in the U.S.: 15 million. By multiplying this number by the penetration rate for an app, you can get a pretty reasonable idea of the number of users that app actually reaches. For Tap Tap Revenge, for example, 15 million multiplied by the 32 percent penetration rate equals 4.8 million users. A figure like this can greatly help marketers and developers come up with more reliable estimates for mobile advertising and premium service revenues.

The other nugget in the comScore report is a chart comparing the amount of time app users spend looking at specific types of content with the amount of time average internet users spend looking at it. It turns out iPhone users exhibit higher engagement with content categorized as retail, conversational media (social networking and blogs), entertainment, sports and search sites vis-a-vis online users. This is rich data for mobile marketers — particularly the figures showing high engagement with retail sites. To my best knowledge, mobile commerce has yet to gain much traction. But based on this data, it looks like iPhone users actually do buy products via their handsets, and want to do so more often. As we’ve reported, advertising targeted to iPhone users has gained momentum in recent months — and rightfully so, it seems.