Shares of fell more than 12% Thursday after a Wall Street analyst labeled the company “the Harvey Weinstein of social media.”

In a note to investors, Citron Research founder Andrew Left — who is a noted short-seller — cited a study released this week conducted by Amnesty International that found Twitter was “a toxic place for women,” particularly women of color.

According to an analysis by Amnesty International and Element AI of a crowdsourced review of 14.5 million tweets posted in 2017, about 7% of the tweets targeted at 778 prominent women in government and journalism in the U.S. and the U.K. were or abusive or otherwise problematic. Women of color were 34% more likely to be targets than white women, and black women were 84% more likely than white women to be mentioned in problematic tweets, the study found. (The study did not look at abuse directed toward men.)

In his research note Thursday, Citron’s Left wrote that “we immediately knew the stock had become uninvestable and advertisers will soon be forced to take a hard look at all sponsorships with Twitter.” He added that “this story has just begun and advertisers will be forced to make more morality-based brand building decisions.”

Twitter trails Facebook and Google in growth, and “any form of tweaking of the business model to ‘monitor speech’ sends traffic, engagement, and total users backwards for Twitter — a death blow for a company playing catch-up,” Left wrote. “Twitter now not only faces the problem of conservative backlash but more importantly the ad buyer who must be sensitive to all social norms.” Citron set a long-term price target of $20 per share on Twitter; the stock was down 12.5% in midday trading, to $28.81 per share. [UPDATE: Twitter shares closed at $29.29 per share for the day, down 11.05%.]

The selloff of Twitter stock also comes amid an investor backlash against Facebook, whose stock dropped 7.3% Wednesday. That came after the attorney general for Washington, D.C., sued Facebook on Wednesday, alleging the company failed to protect the private data of millions of users that wound up in the hands of Cambridge Analytica, a political consulting firm that used the information to target voters during the 2016 presidential election. In addition, a New York Times report Tuesday said Facebook had shared access to user data with more than 150 partners in ways more extensive than previously disclosed, which added to its mounting privacy issues. “As an investor, if you dislike Facebook you must absolutely HATE Twitter,” Citron’s Left wrote.

Asked for comment, a Twitter rep referred to a response to the Amnesty International report by Vijaya Gadde, Twitter’s legal, policy and trust, and safety global lead. In part, she said it was “unclear” how the organization defined or categorized “problematic” content.

“We work hard to build globally enforceable rules and have begun consulting the public as part of the process — a new approach within the industry,” Gadde said in the statement. “As numerous civil society groups have highlighted, it is important for companies to carefully define the scope of their policies for purposes of users being clear what content is and is not permitted.” She added that Twitter welcomed further discussion about how Amnesty International defined problematic behavior “in accordance with the need to protect free expression and ensure policies are clearly and narrowly drafted.”

Over the past several years, Twitter has acknowledged the problem of widespread abuse and other problems like spam on its platform and has stressed that it’s taking active steps to improve the “health” of discussions. Twitter says it uses a combination of machine learning and human review to evaluate abuse reports and whether they violate its rules.

Investor concern over Twitter’s exposure to abuse on the platform comes after the company posted its fourth straight quarter of profitability. During the third quarter of 2018, Twitter reported a revenue increase of 29% year-over-year thanks to strong ad sales.

Twitter’s worldwide average monthly active user count for Q3 dropped by 9 million, to 326 million, while MAUs in the U.S. declined 1 million sequentially, to 67 million. The company said that was a positive sign that it was successfully pruning spammers and other suspicious accounts from Twitter.

“We’re achieving meaningful progress in our efforts to make Twitter a healthier and valuable everyday service,” CEO Jack Dorsey said in announcing Q3 results.