“Angry Birds” was the biggest app of 2011, followed by Facebook, according to a new study. The ranking comes as Distimo estimates there are one million apps for mobile devices now available in seven major app stores.

It was also reported that Apple's App Store is the biggest of the seven app stores. But it has the slowest growth, the study said. “It was the only store not to at least double in size over the past year,” according to a report on InformationWeek.

Some 100 million apps were downloaded from the Mac App Store within the past year, says TechZone360.

On the other hand, the Windows Phone Marketplace increased the most, by more than 400 percent over the last year, InformationWeek adds.

When it comes to games, Microsoft found itself in fourth place following the iPhone, Android, and iPad app stores. Then comes Amazon's Appstore, in fifth place.

One trend that was noticed was the increasing importance of the “freemium model,” says InformationWeek. This is where apps are no-cost to the user but those who want more content or other features need to pay the company. About half of the revenue from 200 “top grossing” apps in the Apple App Store for iPhone comes from these “freemium apps,” the study said. Some 65 percent of revenue from “top grossing” apps come from “freemium apps” in the Google Android Market, the study adds.

The study also found that the Apple App store for iPhone earns some four times the revenue that what is earned by Google’s Android Market, according to Distimo. And more users in China are downloading from the Apple App Store for iPhone, the Distimo study adds.

Some of the one million apps are actually identical titles but designed for more than one platform, InformationWeek explained.

The report from Distimo reviewed: the Amazon Appstore, Apple App Store for iPad, Apple App Store for iPhone, BlackBerry App World, Google Android Market, Nokia Ovi Store, and Windows Phone 7 Marketplace.