After after five years of nonstop growth, China’s most popular social network is shedding users. Often referred to as “China’s Twitter,” Sina Weibo dropped from 309 million users at the beginning of 2012 to 281 million at the end of 2013, according to a study from the government-backed China Internet Network Information Center. The 28 million lost users represent a population more than twice the size of China’s capital, Beijing, and account for 9 percent of the network’s total users.