Contributing to Sina's (SINA +3.3%) gains today may be a report from a government think tank claiming the number of registered users for Sina's Weibo microblogging platform rose 296% in 2011, to 249.9M - a figure that represents nearly half of China's estimated 513M Internet users. Weibo, which has occasionally raised the ire of regulators, was estimated to have 250M users as of November.