Sina Corp, China's largest internet portal and media website, has swung to a loss in the first three months of the year.
Its first quarter net loss was $13.7m (ï¿¡8.6m) compared with a profit of $15m in the same period last year.
The company said advertising revenue jumped 9%, sending its shares up 10% after hours.
However that was outpaced by rising costs, mostly on its Twitter-like microblogging site Weibo.
Weak market
Sina makes most of its revenue from online advertising and analysts said the Chinese online advertising market has been softening.
Sina also blamed slowing economic growth in China for its losses.
"Our brand advertising business got off to a relatively slow start in the first quarter due to the softening of macroeconomic conditions in China," said Charles Chao, Sina's chief executive.
Sina said most of its spending was on hiring employees and setting up infrastructure for its micro-blogging business.
New rules
The company also said it would continue to invest in Weibo's potential.
"The initial feedback from advertisers on our Weibo advertising is encouraging, and we believe it is critical that Sina continues its significant investments in social media and related initiatives," said Charles Chao, Sina's chief executive.
Weibo already has 300 million users.
Recently Chinese authorities disabled the commenting function on microblogs, including Wiebo, for three days. Officials criticised the spreading of "unfounded" rumours.
In Beijing, users now have to register with real identities to post online.
