Weibo Corp stock price has more than doubled this year as steady user growth and an improved sales outlook have lured investors to Chinese social media. But, short sellers are becoming increasingly convinced the stock has peaked.

Bearish bets on Weibo, China's answer to Twitter Inc, are at the highest ever, even after the company reported earnings that beat analysts estimates and forecast a bigger-than-expected increase in third-quarter revenue. The Nasdaq-traded stock posted a 19 percent gain last week, while the number of shares borrowed for short selling rose to a record 4 million.

Bullish investors are betting that television and video partnerships the company has been forging will further boost the number of people using Weibo, which has been growing at a steady rate of about 30 percent annually for the past two years. Short sellers are wagering that a 119 percent rally this year, twice as much as the second-biggest gainer among Chinese stocks in the United States, has become excessive.