As Facebook prepares to go public it has its sights on China, where the social media titan is blocked, but analysts say its chances of re-entering the market of half a billion Internet users are slim.

Facebook, which filed paperwork on Wednesday seeking to raise $5 billion on Wall Street, said it continued to 'evaluate entering China' - the world's largest Internet market and a huge dark spot on Facebook's global map.

"There are more than two billion global Internet users and we aim to connect all of them," the California-based company said in the listing documents filed with the US Securities and Exchange Commission.

Facebook, which has more than 800 million users around the world, is the leading social network in all but six countries, notably Russia, where local rivals are preferred, and China, where it has been banned for years.

Beijing blocked Facebook along with micro-blogging site Twitter after blaming it for fanning social unrest in the northwestern region of Xinjiang in 2009, though many web users access the site via virtual proxy networks.

Prior to the ban, web users in China enjoyed uncensored access to Facebook.

In the run up to a major leadership transition later this year, experts said Facebook has next to no chance of being allowed to operate freely in China, unless it is willing to bow to the country's army of censors.

"Under the current political structure (it has) probably zero to nil" chance of operating in China again, said Bill Bishop, a Beijing-based Internet consultant and investor.

Anne-Marie Brady, an expert in Chinese politics at New Zealand's University of Canterbury, agreed, saying the only way Facebook could re-enter China was if it followed "the same censorship requirements as Chinese-based websites".

Despite the hurdles, Facebook Chief Executive Mark Zuckerberg is clearly keen to see his company crack the Chinese market, which has more than 500 million users, nearly half of whom use weibos, which are similar to Twitter.

Zuckerberg said previously that he was "spending a lot of time" studying Chinese and visited the country in December 2010 with his girlfriend Priscilla Chan, when he met with the head of the country's biggest search engine Baidu.

Analysts believe Zuckerberg and Baidu Chief Executive Robin Li were discussing a possible tie-up that could enable Facebook to enter China, but were forced to abandon the talks after the Arab Spring protests in early 2011.

"Facebook was used heavily by protesters during the upheaval across North Africa and the Middle East, underscoring Communist leaders' fears that these sites posed as a 'real threat to stability and the government'," Bishop said.

"Ultimately it was realised by both parties that the Chinese government was not going to approve Facebook in China," he said.

"The government is very cognitive of the power of these networks and the potential threats to the government."

But caving in to Chinese censors did not make business sense either, Bishop said. Facebook would have no 'competitive advantage' over home-grown social networking sites such as Sina's Weibo, Tencent's QQ and Renren.

Beijing attempts to block content it deems politically sensitive through a censorship system known as the "Great Firewall of China".

The next 12 months are particularly sensitive for Chinese leaders as they prepare for a once-in-a-decade transition of power that begins later this year, and Beijing is stepping up efforts to keep a lid on social unrest.

Internet and technology firms have been pressured to stop the 'spread of harmful information', while Beijing, Shanghai and the southern province of Guangdong have ordered weibo users to register under their real names, making it easier for authorities to track them down.

But the explosion of social networking sites poses a huge challenge to official attempts at controlling information in the country of 1.3 billion, as users can re-post news and images as fast as censors take them down.

The number of weibo users more than trebled in 2011, jumping to 250 million from just 63 million at the end of 2010, official data shows, as more and more people went online to vent their anger at official corruption and scandals.

A weibo user is believed to have broken the news of a deadly high-speed train crash in July that provoked widespread condemnation of the government, while news of a rare revolt against Communist officials in the country's south in December first emerged on weibos.

Amid the growing influence and popularity of social networks, Facebook will struggle to convince authorities obsessed with maintaining stability to unblock its website, said Duncan Clark, chairman of Beijing Internet consultancy BDA.

"In terms of 2012 I would think it unlikely that the government would send a signal of liberalisation during a season of conservatism and control ahead of the leadership transition," he said.