Facebook and Twitter are blocked by China's great firewall so it was only a matter of time before something came up to fill the void. Sina, a leading Web portal here in China has a microblogging service called Sina Weibo, the company's Twitter-like social-media service. According to Sina says Weibo users doubled to more than 100 million just in the last four months of 2010. Last month, Sina added instant messaging and location-based services to Weibo, which the company now says is the centerpiece of its media growth strategy. To give you an idea of why you need to know about Sina, China ended 2010 with 457 million Internet users, up more than 19% from 2009, according to China's government. The U.S. ended the year with 211.9 million Internet users, up just 3%, says market tracker ComScore. Here is a look at an early build for Sina running on MeeGo.