(Sarahah was launched in the Middle East this past February, and it's already said to have millions of users worldwide.Sarahah)

There's a new app beating out YouTube, Instagram, and Snapchat at the top of the App Store — but most people in the US have probably never heard of it.

It's called Sarahah, and it's an app for sending and receiving feedback from friends and co-workers anonymously.

The app, which was built by Saudi programmer Zain al-Abidin Tawfiq and named after the Arabic word for frankness or candor, launched in early February. By the end of the month, BBC reported the app already had 2.5 million users in Egypt, 1.7 million in Tunisia, and 1.2 million in Saudi Arabia.

Sarahah arrived in Apple's App Store in June and, in the last few days, has skyrocketed to No. 1 among the App Store's top free apps.

So, how does Sarahah work? And why do its App Store reviews describe it as "a breeding ground for hate"?

'I don't recommend going on here unless you wish to be bullied'

The app is designed to help you in discover your strengths and weaknesses "by receiving honest feedback from your employees and your friends in a private manner."

Though it's tough to tell much from Sarahah's website, and the app itself doesn't reveal much, it seems as though Sarahah lets you share your user name with whoever you want so they can send you anonymous messages (the app recommend attaching a link to your username on Snapchats). While you can favorite messages, you can't respond to them.

While Sarahah appears in Arabic on the App Store, the text is in English once the app is downloaded to your phone.

But Sarahah has mixed reviews in the App Store, with some calling it the perfect app for bullying.

"My son signed up for an account and within 24 hrs someone posted a horrible racist comment on his page including saying that he should be lynched," one user posted. "The site is a breeding ground for hate."

"I don't recommend going on here unless you wish to be bullied," another user wrote.

"Parents, don't allow your kids to get this app," a third user posted. "This is an app breeding suicides."

(Sarahah bears a resemblance to an early version of Yik Yak, an anonymous messaging app that shut down this year.iTunes)

But Sarahah isn't the first anonymous messaging app, and it's not even the first anonymous messaging app to be accused to racism, threats, and bullying. In 2015, threats found on the college-focused messaging app Yik Yak prompted a criminal investigation at a college campus. The same year, a college junior was suspended from school for six months after posting a racist comment about black women, and another student was charged with a hate crime for posting "Let's lynch her" about another student on the app.