Four of the top 10 iPhone apps are stock iOS apps, as YouTube and Maps are likely referring to the versions of the app bundled in with iOS 5 and earlier rather than the newer versions that launched in the past couple of months, as the new versions likely wouldn't have had the time to make a significant impact on the statistics. Nielsen notes that Facebook Messenger was the fastest growing iPhone app.
In fact, Facebook controlled three of the top 10 in Facebook, Facebook Messenger and Instagram while Google, at this point, controlled none of the top 10.
Maps averaged 32 million unique users a month while YouTube averaged 22 million, Stocks averaged 21 million and Weather averaged 20 million uniques a month.
The study also found that Android sits atop smartphone marketshare with 52% with iOS in second at 35%. Blackberry OS came in third at 7%, Windows Phone with 2% and the rest made up the remaining 5% of the marketshare pie. The statistics come from Nielsen's Smartphone Panel, which uses on-device meters at the aggregate level to find out mobile web and app activity level.