Sina (SINA.NASDAQ), which operates China’s third-most visited website, posted a first-quarter profit which missed estimates as it spent to promote its new microblogging service Weibo, Bloomberg reported. Net income fell to US$15 million, down from US$24.4 million a year earlier. Analysts had expected a figure of US$15.4 million. Sales increased by 18% to US$100.2 million during the period, but Sina is being forced to spend heavily on advertising, promotion and development of its Weibo service to fend off competition from other well-funded rivals like Tencent (0700.HK). Operating expenses rose 25% to US$41.8 million. CLSA analyst Shi Jialong said that Weibo is unlikely to make any significant contribution to Sina’s revenues this year. Sina’s shares slipped 29 cents to US$119.80 on NASDAQ prior to the announcement, but the stock is up 74% this year, making it the best performer on the Morgan Stanley Internet Index.