Since it first opened its doors to the general public back in July 2006, Twitter has expanded rapidly around the world, quickly establishing itself as the de facto micro-blogging network in almost every major country.

In August we looked at a study that suggested, perhaps unsurprisingly, that the bulk of Twitter’s 500 million (and growing) registered membership was located in the USA, with some 141.8 million users.

But what about active users? Which country heads that table?

Well, according to eMarketer, citing a GlobalWebIndex report, it’s… China.

Yep. That China. The one that’s been blocking since June 2009. Despite this obvious problem, the GlobalWebIndex estimates 35.5 million of Twitter’s 140+ million active Twitter users are to be found in China. Or, to put it another way, one quarter. Of everyone. On Twitter.

Which, I guess, says a lot about how important all of us feel it is to be privy to Twitter as an open source of free information, irrespective of the risks (which, in China, can be considerable).

Logistically, of course, this makes sense, as China is the biggest country in the world, so it should be number one on Twitter, too. But in reality it’s very much a surprise, simply because the country – or its government at least – has been so hostile towards the top internet sites in the West, which includes social media (Facebook is also blocked).

The U.S. wasn’t even second – that placing belongs to India, which only narrowly trails China at 33 million active users. The U.S. (22.9 million), Brazil (19.6 million) and Mexico (11.7 million) round out the top five.

The GlobalWebIndex defines an active Twitter user as somebody who has used or contributed to the platform at least once in the past month using any device, which is exactly the same way as Twitter does it. So, surprising as these numbers might be – and it’s important to remember that they are estimates – there might just be something to them.

(Source: eMarketer.)