For Li Mo, the footage of black-clad people clashing with police and vandalizing storefronts proved the final straw. The images of Hong Kong protesters fighting for greater autonomy from Beijing incensed the mainland-born postgraduate student and she could no longer remain on the sidelines. So, she joined China's fangirl army.

Ever since anti-government demonstrations in Hong Kong turned violent this summer, China's celebrity-obsessed young generation have patrolled Facebook, Twitter and Weibo, ready to pounce on perceived slights and defend their motherland. Nicknamed "fangirls" because they exhibit the same fervor most often reserved for pop-culture icons, these women and men flood social media with slogans and memes shaming brands — sometimes with far-reaching consequences.

Fangirls called out Houston Rockets General Manager Daryl Morey for supporting Hong Kong protesters, prompting China's state broadcaster to drop National Basketball Association games. They triggered boycotts of brands from Coach to Apple. Many got swept up by Facebook and Twitter account takedowns. And in a recent incident, the onslaught of vitriol they directed at Hong Kong pop-star Joey Yung forced her to apologize for a single Facebook selfie, but not before she got canned from a high-profile gala.