Citron Research's Andrew Left is betting against Twitter less than two months after the short-seller made an unusual and public bullish bet on the social media company.
Left, who has gained notoriety for successful bets against stocks such as Valeant Pharmaceuticals, explained that his rapid change of view on Twitter came as a result of the growing scrutiny surrounding social media and how companies handle users' personal information.
"Everything's changed; everyone is talking about data privacy," Left told CNBC. "They're a lot more vulnerable than Facebook. ... If you're involved in the market you've got to stay aware of the news."
Shares of Twitter fell more than 12 percent Tuesday into bear market territory following Left's tweet announcing the about-face. He also told CNBC that Twitter's steep valuation and minimal short interest also affected his decision.
"Citron short Twitter," Left tweeted. "Near-Term target $25 Of all social media, they are most vulnerable to privacy regulation Wait until Senate finds out what Citron has published."
In response, Twitter issued the following statement: "To be clear - our data licensing business does not sell DMs. Any reports to the contrary are wrong."
In its biggest crisis ever, Facebook is under fire over its handling of personal data following reports that political research firm Cambridge Analytica wrongly gained access to personal data of more than 50 million Facebook users. Facebook shares are down more than 11 percent in March as the scandal unfolds.
To be sure, Left's quick pivot on Twitter comes less than two months after he told CNBC he had bought shares and had a bullish outlook on the company.
Despite being much maligned, Twitter is "a compelling product," Left told CNBC in January. "When you compare the market cap to Facebook, it has a long way to go. … It's a compelling platform for where it's trading."
— CNBC's Tae Kim contributed to this report.