<p>BEIJING (Reuters) - Facebook Inc and Twitter face a daunting task in China, if access to their social networks is unblocked, as they would be up against deeply entrenched domestic rivals which cater to local needs and tastes.</p><p>Years of isolated growth means China's sophisticated social media companies, including Tencent Holdings, Sina Inc and Renren Inc, won't be too worried if Facebook and Twitter prise open the door to China's 591 million Internet users, the world's biggest online population.</p><p>Access to Facebook and Twitter has been blocked in China since 2009, but will be lifted by the government in the Shanghai Free Trade Zone (FTZ) which is due to launch this weekend, the South China Morning Post reported on Tuesday - a move that has been popularly dubbed the "Internet Concession".</p><p>But it may be too late for them to repeat their success elsewhere in one of the world's most promising, yet most restricted, Internet markets - where online advertising revenues soared almost 47 percent last year to $12.3 billion.</p><p><a href="http://www.reuters.com/article/2013/09/25/net-us-china-internet-idUSBRE98O09N20130925">Keep reading...</a></p><p>Read also:</p><p><a href="http://www.nbcnews.com/technology/facebook-twitter-will-be-unblocked-chinas-free-trade-zone-report-4B11243907">Facebook, Twitter will be unblocked in China&#039;s free-trade zone: report</a> (NBCNews.com)</p><p><a href="http://variety.com/2013/digital/news/facebook-twitter-to-be-permitted-in-shanghai-says-report-1200664738/">Facebook, Twitter to be Permitted in Shanghai, Says Report</a> (Variety)</p><p><a href="http://qz.com/127615/china-will-unblock-facebook-twitter-and-the-new-york-times-to-boost-its-new-free-trade-zone/">China will unblock Facebook, Twitter and The New York Times to boost its new ...</a> (Quartz)</p><p>Explore: <a href="http://news.google.com/news/more?ncl=dLR4BhyGGBrULKMvg5zz9zWHasMPM&ned=us">148 additional articles.</a></p>