A new study by Distimo finds that social networking apps have grown much faster than any other category in the last two years. U.S. downloads of social apps on the iPhone have surged 193% in that period compared to 43% for top 100 most popular apps overall, according to the app analytics firm. Facebook, with its 543 million monthly mobile users, has played a big part in that trend. But the report shows that newer social media properties are making inroads, especially outside the U.S. Distimo's broad definition of social apps includes those that include communication, media and location-based service. Viber and Skype, for instance, are free calling apps included in the study. During the last two years (July 2010-July 2012), Facebook has been the dominant social app on an aggregate basis among the world's 20 largest countries. But in the last year, Instagram has moved up quickly to become the second-most-popular app. “Instagram could have potentially harmed Facebook's position, which might indicate why Facebook acquired Instagram,” the report concludes. Last month, Instagram was the most downloaded app in the top 10 countries, followed by Twitter, Facebook, Skype and Pinterest. While Facebook remains the top social app overall in Asia, it's no longer the most downloaded in many individual Asian nations. Line was the top social app in Japan, Taiwan and Singapore, while WeChat was the most downloaded in China and Hong Kong in the last year. KakaoTalk was No. 1 in South Korea. Outside Asia, Facebook was supplanted by Viber as the top app in Spain and Australia in the last year. Globally, LINE, Facebook Messenger, Google+ and WeChat have broken into the top 10 in the last year, while Windows Live Messenger has dropped out. Among paid social apps, Whatsapp was the top brand. If it offered a free version, Distimo suggests it would be one of the most popular social apps worldwide.