Mobile marketing continues to grow in importance for advertisers, and as such Nielsen's made mobile usage statistics a major part of its data reporting. Its latest report focuses on how consumers use apps on their smartphones, and amid some relatively unsurprising statistics (the number of iOS and Android users in the US more than doubled in the last year, from 38 million to 84 million, for example) was a note on the top five apps smartphone owners use. Somewhat surprisingly, the top apps were Facebook, YouTube, Android Market, Google Search, and Gmail — the same as Nielsen saw last year. While Facebook and YouTube aren't terribly surprising, the other three apps are rather unexpected — the sizable population of iOS users obviously isn't accessing the Android Market, and the Google Search and Gmail apps on iOS are unspectacular, at best.

On the surface, there's a few potential issues with this data. For starters Nielsen doesn't make it clear how this data was gathered — the company told us it sourced data from several sources, including a monthly "Mobile Insights" survey and an in-depth survey of those who report having downloaded apps in the past thirty days. Additionally, Nielsen also has a panel of 5,000 iOS and Android users who have agreed to let their mobile usage be measured. Depending on where this data was sourced from, there could be a number of issues with it. If Nielsen's survey asked respondents what their top five apps were, there could be some ambiguity skewing the results — "Google search" could refer to the app itself, or searching Google in the browser, or searching using the Google search widget that's built into Android 4.0. Similarly, "Gmail" could potentially refer to Google's official Gmail apps, or checking Gmail through the default iOS mail app (or third party options like Sparrow), or even checking Gmail through the browser.

We'd love to know more about how Nielsen determined its top five apps

The inclusion of Android Market in the top five apps appears particularly unusual — while the Android Market / Google Play is obviously used frequently for installing and updating apps, it would be surprising if people downloaded it more or spent more time using it than highly popular cross-platform apps like Instagram or Angry Birds. And while some studies show that Android marketshare has surpassed that of iOS in the US, we'd expect the iOS App Store to have similarly high usage rates compared to the Android Market. Is it possible that Google Play's background app updating process skewed Nielsen's results?

Lastly, four of the top five apps are included with Google-approved Android devices (and YouTube is a stock option for iOS as well) —even if these are the most-used apps for all smartphone users, it tells us nothing of the vibrancy of the third-party smartphone app landscape. Given the massive variety of apps available to consumers, app preferences are likely becoming increasingly diverse — Nielsen's own data shows that the proportion of time spend on the "top 50 apps" declined from 74 percent in 2011 to 58 percent in 2012. Even so, there are always apps that rise to the top, like Facebook, and it would be a lot more valuable to see how those third-party options (rather than built-in default apps) stack up. We've reached out to Nielsen to find out more about how it measures top app performance and will share any details the company is able to share with us.