HANOI (Reuters) -  This isn t like China,  says Vietnamese activist  Anh Chi  at a noisy bar off one of the narrow streets of Hanoi s Old Quarter.  They can t shut Facebook down here.  His 40,000 Facebook followers make him one of Vietnam s better-known critics, but by no means the biggest in a Communist state whose attempts to crack down on dissidents have collided with the rapidly expanding reach of foreign-owned social media. Vietnam s President Tran Dai Quang this month called for unspecified tougher internet controls in the face of  hostile forces  that he said threatened not only cybersecurity but also  undermined the prestige of the leaders of the party and the state . But taming the internet in a young, fast-growing country is not easy, especially when the companies providing the platform are global. China, in contrast, allows only local internet companies operating under strict rules. Vietnam is among Facebook s top 10 countries, by number of members. It now reports more than 52 million active accounts to advertisers, according to research provided to Reuters by social media agencies We Are Social and Hootsuite. Google s YouTube and Twitter are popular too. As elsewhere in Southeast Asia, social media underpins business and communications as well as government critics. Some dissidents posting on social media have been caught in a major crackdown that has followed changes in the ruling party hierarchy. At least 15 people have been arrested this year. High profile bloggers Nguyen Ngoc Nhu Quynh, known as  Mother Mushroom , and Tran Thi Nga have been jailed for 10 and nine years respectively. Government critics also complain of beatings by unidentified assailants and intimidation. But dozens of activists still post critical comment every day. Several have more than 100,000 followers and at least one has over 400,000 - more than double that for the government s own Facebook page and nearly a 10th the size of the Communist Party s national membership.  We use any chance we have to raise our voice: environmental issues, territorial issues, land issues,  said  Anh Chi , 43, a Vietnamese teacher, translator and publisher whose real name is Nguyen Chi Tuyen. Vietnam tried to pressure Facebook (FB.O) and Google (GOOGL.O) to take down thousands items of anti-government content in March by leaning on advertisers, but the continued prevalence suggests limited success. One reason it is hard to take tougher action is business: From brewers to insurers to the makers of the motorbikes buzzing Vietnam s streets, social media is a key marketing route to young and increasingly affluent consumers in an economy growing at more than 6 percent a year, one of the fastest rates in Asia. For small businesses it is crucial: One new silk flower shop in Hanoi told Reuters 95 percent of customers found it through Facebook or Instagram.  You ve got kids that are building businesses on these platforms and generating significant success,  said Simon Kemp, founder of the Kepios marketing consultancy. While it accounts for only a tiny part of Facebook or Google parent Alphabet s revenue, Vietnam is a hot target for global consumer brands. Asia-Pacific was Facebook s fastest growing region by revenue last year, up nearly 60 percent. Tighter Internet controls could dampen innovation and impact the growth of Vietnam s digital economy and its competitiveness, said Jeff Paine, managing director of the Asia Internet Coalition, whose members include Facebook, Google and Twitter. Facebook did not respond to a request for comment. Google declined to comment. Vietnam s foreign ministry spokeswoman Le Thi Thu Hang told Reuters the government was an advocate of the internet but tried to minimize  behaviors that harm users and illegal acts such as inciting violence and a depraved lifestyle . China blocked Facebook in 2009 and only local sites such as WeChat and Weibo are permitted, operating under laws that ban content that is obscene, violent or offends the Communist Party.  China has had remarkable success controlling discussion,  said David Bandurski, co-director of the China Media Project and a fellow of the Robert Bosch Academy in Berlin. Tools include keyword filtering by the local internet companies and the close monitoring of big networks, he said. Even so, China has said it is investigating its top social media sites for failing to comply with its laws. Facebook has been blocked in Vietnam occasionally - sometimes at sensitive moments - but never for long.  Vietnamese authorities have tried for years and so far failed to stop independent journalists and bloggers from using the internet,  said Shawn Crispin, Southeast Asia representative of the Committee to Protect Journalists.  It s a losing battle.  That does not stop activists being targeted for arrest. Activist Pham Doan Trang noted on Facebook that some campaigners appeared to have withdrawn from the scene in the face of the crackdown, but said she would not be discouraged.  Freedom has a very funny rule,  she told Reuters  Once people know the limit of freedom they will never go back.  