You may never have heard of Yubo before. It's certainly not as big as Facebook, YouTube, or Instagram. The social networking app isn't even usually listed alongside fairly new social platforms that have become extremely popular with Gen Z, such as TikTok, Twitch, or Discord.

But now, Yubo has been thrust into the mainstream. Why? The Uvalde school shooter was reportedly a user on the platform. And, according to those who interacted with him on Yubo, the gunman showed warning signs.

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But, what is Yubo?

Formerly known as Yellow, Yubo is a social media platform that mixes livestreaming with social networking. Based out of Paris, France and founded in 2015, it's a mobile-only platform, meaning that it requires an iOS or Android device in order to join the network. 

Yubo's user base is made up mostly of teens and young adults. According to TechCrunch, the company says 99 percent of its users are part of Gen Z and range from 13 to 25 in age. Mobile analytics firm Sensor Tower estimatesthat the Yubo app has been downloaded more than 18 million times in the U.S.

For a while, Yubo earned the tagline of "Tinder for Teens." Yubo's developers first developed the app to connect Snapchat users after realizing young people were looking to make new connections on the platform. Snapchat is generally meant as a social platform for users and people they already know. Users and their posts are not publicly searchable like they would be on platforms like Twitter or Facebook.

Noticing an opening in the marketplace, the Yubo team created an app where Snapchat users could connect with strangers by swiping left or right, like on Tinder. The idea soon evolved into a platform of its own now known as Yubo.

While news that the Uvalde school shooter used the app is bringing Yubo unwanted attention, it's not the first time Yubo has been put in the spotlight. As Protocol pointed outin 2021, Yubo has generated lots of concern among parents of its target audience.  CBS's local news affiliate in Tampa Bay is among media outlets that aired a segment warning parents about the new app kids might be usingin 2020. Business Insider reportsthat a 26-year-old adult man was arrested in 2019 after trying to meet a 12-year-old girl that he met on the app.

According to Yubo, the platform attempts to keep its underage users and adult userbase separate on the platform. The company announcednew age verification techniques just one day after the shooting, before the Uvalde gunman's presence on the platform was publicly known.

Earlier this week, Yubo toldBusiness Insider that it's consistently working on features and protocols to ensure the safety of its users, such as AI-powered moderation of livestreams and chats. The platform also doesn't display advertisements or show users algorithmically-promoted content, two features usually found on social apps that can have a negative impact on teenagers.

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However,  The New York Times, Washington Post, and VICEall spoke to Yubo users who had interacted with the Uvalde shooter, Salvador Ramos, on the app. One young girl said Ramos asked her to be his girlfriend on Yubo, which she described as "Tinder for kids." According to the girl, Ramos became aggressive after she turned him down.

The New York Timesspoke to a 15-year-old girl in Germany whom Ramos met on Yubo. The teen girl said Ramos sent her pictures of his guns and told her about his plans on the day of the shooting. However, she did not believe him.

According to one 16-year-old boy who spoke to the Washington Post, Ramos frequently made hostile remarks on the Yubo platform. He said that the shooter frequently posted photos of dead cats and threatened girls on the app with sexual assault and rape on Yubo's text message and live group chat features. He said that Ramos' account was often reported to Yubo for his comments, but the platform failed to ever act, allowing Ramos to continue to send threats to other Yubo users.

In addition to Yubo's failure to respond to Ramos' actions on the platform, the shooter's Yubo account remainedlive on the platform until just Saturday, nearly 4 days after the shooting that left 21 people, including 19 students, dead.