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It's home to the world's biggest internet population, many of them mad keen on social networking. But China blocks access to Facebook, Twitter and YouTube.

That means many of the 500 million internet users are turning to home-grown firms, which are building giant social media businesses.

Among them is Tencent, which offers everything from the instant messaging service QQ to one of the leading Weibo networks that replicate much of what Twitter does and more.

The company is also investing hefty sums in online video channels, which now produce hours of programming for internet users.

When we visited the tower block in Beijing's technology district where this video operation is based, I asked how many floors were occupied by Tencent. "Oh, all of them," I was told.

The business now employs 20,000 people across China on its various social media operations - compare that with the 1,500 working for Twitter or the 4,000 employed by Facebook and you get a sense of the scale of a business making substantial revenues from advertising.

Animated election

On the ground floor, my Tencent hosts showed me around the nearly completed state-of-the-art studio complex that will soon allow the simultaneous broadcast of six online programmes. Then, upstairs we found the existing rather cramped studio that is Tencent Video's current base.

The whole operation is integrated with the QQ and Weibo sites, so that online viewers take part in the programmes - what's becoming known as a second-screen experience.

As the studio team prepared for a regular financial slot, they were keen to show off their most high-profile series of recent months, .

The jokey graphics opening sequence, with a cartoon Osama Bin Laden popping up, set the tone of a programme that attracted online audiences of up to 40 million.

It seems Chinese social media users were eager to join in discussions that have often been taboo at home.

"We had items on attitudes to sex and immigration, and how they compare in the US and China - people liked it," the young producer, Cho Ye, said.

The editor Blei Zhang, a casually dressed man in his early 30s, believes Tencent is part of a social media revolution opening up debate in China.

He said: "Netizens, as we call them, can discuss and share more of each other's thoughts and opinions. And that will help China become more open."

There is clear evidence services like Tencent Weibo and others like RenRen - China's version of Facebook - are allowing discussion of previously taboo subjects.

And the fact that they are used all over China, by people young and old, means that stories and scandals once kept hidden in some provincial town are now getting nationwide coverage. But there are limits to this new wave of self-expression.

'Red line'

I visited one of China's most adept users of social media to discuss this.

Wang Keqin is a former investigative journalist who now runs a campaign to help migrant workers with health problems.

It has attracted huge interest, and he now has nearly seven million followers across four different Weibo services for his "tweets" about the suffering of the migrants, and the iniquities of the authorities who ignore their plight.

Mr Wang told me he could be quite outspoken - but there were limits. Sometimes the editor of a Weibo site rang him to ask him to take something down, and sometimes he censored himself.

There was a "red line" beyond which he would not go.

"It's intangible but being educated under the rule of the Chinese Communist Party, I normally know where this red line is," he said.

"So, I can't publish comments critical of the party, but I can do satire or sarcasm."

Internet law

And, at Tencent HQ, we also found evidence this new spirit of free expression in social media only went so far.

I asked Blei Zhang whether people commenting on his programmes via QQ or Weibo could say anything they liked. "Sure," he responded.

But what about if they wanted to criticise the Communist Party. There was an awkward pause, and then the interpreter intervened: "This is something sensitive."

Eventually, Blei Zhang explained that people could say anything they liked - as long as it complied with China's internet law.

Some people do get around the blocks on overseas social networks. Recent figures suggest 35 million people use Twitter, despite the ban.

When I talked to a group of Beijing medical students, one told me he used virtual private network (VPN) software to get access to Facebook, but that it was too much of an effort for most people to bother.

Others seemed satisfied with the likes of RenRen, Weibo and QQ, but were aware that there were rules on just what could be said.

"If someone talks about sensitive things, if they don't have the right opinion, they will be kicked out," said one. "We have to be careful."

But questions about politics and freedom of expression are probably not uppermost in the minds of executives at Tencent and its rivals. They are riding China's social media boom and they believe it has a long way to go yet.