Andrew Left, the noted short-seller, has targeted Twitter Inc. (TWTR) - Get Report  .

Twitter shares were falling 12.5% to $28.78 on Thursday after Left, founder of Citron Research, published a quick note saying that "Twitter has become the Harvey Weinstein of social media." 

In other words, he's saying Twitter needs to clean up its platform, a process that will put pressure on revenue. Left's new price target is $20, 31% below its current level. 

Left mentioned that Amnesty International, a non-governmental organization, has deemed Twitter abusive to human rights and called it "toxic" for women. 

Not only would a potentially much-needed platform clean-up hurt user growth and engagement, but something else would happen even before users leave the platform: Advertisers that need to be socially sensitive might flock too.

"Any form of tweaking of the business model to 'monitor speech' sends traffic, engagement, and total users backwards for Twitter," Left wrote in his note. Plus, "Twitter now not only faces the problem of conservative backlash but more importantly the ad buyer who must be sensitive to all social norms," Left said. 

Left previously targeted Snap Inc. (SNAP) - Get Report as a reeling social media company. Snap, among other companies, is a stock he has shorted. 

Twitter has risen 19.95% year to date.