Google, Facebook and Twitter are all banned in China, but the Internet giants' top executives are increasingly frequent visitors to Beijing as they seek opportunity and profit from the world's second-largest economy, despite concerns over censorship.

Google terminated most of its operations in mainland China in 2010 after controversy over the country's online controls and an attack on users of its Gmail service.

But Eric Schmidt, its former CEO and now president of its new parent company Alphabet, was in Beijing last week declaring: "We never left China."

"We would very much like to serve all of China," he told a Techcrunch forum. "We continue to chat with the government."

Google has kept around 500 employees in Beijing and Shanghai, mainly selling advertising spots to Chinese companies.

The country's Communist authorities operate vast limits on Internet access, dubbed the Great Firewall of China, and exercise formidable censorship within their cyber borders.

Facebook has been blocked since 2009, its photo-sharing subsidiary Instagram has been unavailable since last year's pro-democracy protests in Hong Kong, and Twitter and Google-owned YouTube are inaccessible.

Nonetheless, the CEOs of several Silicon Valley giants lined up to be photographed alongside Chinese President Xi Jinping during his state visit to the United States in September.

When China's top Internet official Lu Wei visited Facebook's headquarters last year he was photographed at CEO Mark Zuckerberg's desk -- with Xi's book The Governance of China next to the billionaire's laptop.

Facebook claims more than a billion active daily users, and China's 668 million Internet users represent a huge potential market -- if access to it is ever unblocked.

Zuckerberg's Chinese-language speech to Tsinghua University in Beijing last year caused a sensation, and he was back last month to remind students of a Chinese proverb urging perseverance: "If you work hard enough, you can grind an iron bar down to a fine needle. With patience, you can change the world."

- 'Internet sovereignty' -

US think tank Freedom House listed China as the world's worst abuser of Internet freedom in a report last month, and campaign groups express concern for Internet freedoms worldwide if firms begin kowtowing to Beijing's strict censorship demands.

China hopes to push for "Internet sovereignty" -- the concept that each country should have the right to control the Internet as it sees fit.

It has promoted the concept at high-profile events such as the "World Internet Conference" it mounted last year.

"We have great concerns that Internet companies may kind of lower their standards and comply with some Chinese practices that are contrary to human rights, especially freedom of expression and users' right to privacy," said William Nee, China researcher for Amnesty International.

Google Maps has once again become accessible from mainland computers, but it shows the nine-dash line delineating Beijing's claim to almost the whole South China Sea and identifies Taiwan as a "province", among other differences to the global version.

- 'Bigger and bigger' -

Chinese state mouthpieces such the official Xinhua news agency and broadcaster CCTV are not averse to Western social media, spreading Beijing's gospel -- in English -- on Twitter and Facebook.

The two firms are also seeking Chinese corporate and government clients. Several Chinese cities have set up Facebook pages to promote themselves to foreign tourists, while Xi's US tour in September had a verified page on the network.

"Twitter is not yet here" but it does have mainland clients, said Alan Lan, Twitter's head of online sales for Greater China, which includes Taiwan and Hong Kong.

"We have a lot of local enterprises whose communication is Chinese but (which) need to think globally: that's where Twitter comes and can help," he told the Techcrunch event.

Jeremy Goldkorn, founder of Danwei, which tracks Chinese media and the Internet, told AFP that Western social media giants were more likely to generate revenue from Chinese advertisers than users.

They cannot hope "to be able to operate in China the same way they do in America and in Europe", he said.

"The only way they will be able to operate is by making quite a lot of concessions to the Chinese government... and I don't think they are ready for it."

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