The most popular mobile apps these days are almost always from huge companies like Facebook, Snap, and Netflix. But this week a relative unknown called Sarahah has rocketed to the top of the charts, becoming the most popular free download from the U.S. iOS App Store, and the second most popular free app on Google Play, according to App Annie.
Sarahah pitches itself as a social network that lets users receive anonymous messages. It was initially introduced as a website by Saudi Arabian developer Zain al-Abidin Tawfiq, who told tech news site Mashable that he wanted to help employees give anonymous, and hopefully more honest, feedback to their bosses. “Sarahah” is the Arabic word for candor or openness.
More from FORTUNE
Sarahah’s spread, according to Tawfiq, was a case study in viral growth. He first introduced it to a few influential friends, and by early this year it had caught on across the Arab world. Tawfiq told Mashable that its appeal lay in its ability to break through traditional social barriers of age and rank.
Get Data Sheet, Fortune’s technology newsletter.
But since the June 13 debut of Sarahah’s English app, its explosive growth has apparently been driven by teenagers, raising a host of concerns about safety. Despite its optimistic premise, users already report that the app has become a breeding ground for cyberbullying and hate speech.
If history is any lesson, that suggests Sarahah’s meteoric rise may prove unsustainable. Anonymous messaging platforms like Yik Yak, Whisper, and Secret have consistently struggled to control malicious behavior, leading to declining use and, for Yik Yak and Secret, ignominious closure.