The answer to the most popular smartphone app might differ from an iPhone to an Android junkie. But, Nielsen’s mobile application report has revealed that Facebook was the most used app on all smartphones including the iPhone and iPod touch, Blackberry, etc (except devices running on the Android platform).

In general, the study reveals an increase in smartphone usage and the apps popular in it.

With 21% of American wireless subscribers owning a smartphone in Q4 2009, the Facebook app had made it to 58% of the iPhones and 51% of the BlackBerrys. Facebook is the top app even among BlackBerry users even if the BlackBerry is perceived to be a business phone and users are typically perceived as businessmen. It was only on the Google-backed Android platform that Google Maps made it to the top spot, with Facebook being a close second (50%).

On average, a smartphone user has 22 mobile apps on their handset and the number of apps in a feature phone is 10. It was observed that social networking, maps, weather and music remained the most popular apps across all platforms.

The Nielsen study polled in over 4,200 people who had downloaded a mobile app in the past thirty days. The study also revealed that an average smartphone has 22 apps in contrast to the iPhone, which had an average of 37 apps. Android had 22, Palm had 14, Windows Mobile had 13 and BlackBerry had 10.

Surprisingly, Windows Mobile did not even get its own graphical breakout by Nielsen. It appears that games are the most popular application downloaded for both smartphones and feature phones during the study. Games are closely followed by news and weather, maps/navigation and social networking for smartphones. However, feature phone users seem to download more music than map/navigation applications. Although Facebook is the clear favorite app, MySpace is hugely popular among teens too and LinkedIn attracts adults in the age group of 25-44.