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A social media messaging app called Yolo has become the most popular app on iTunes in the UK and US just a week after its release.

It's taken media experts by surprise because it has appeared at the top of the chart without any promotion.

Yolo lets Snapchat users request anonymous messages from specific friends or the wider public.

Its success has caused concern because of bullying and abuse on previous anonymous online question and answer (Q&A) services.

Snap - who own Snapchat - has told Newsround that Yolo is not owned by or affiliated with their company in any way.

Yolo - which stands for 'you only live once' - is one of the first apps built by Snap Kit.

Snap Kit was launched by Snapchat last year as a way for smaller companies and developers to get involved within its social network.

Yolo uses Snapchat by letting people post a photo inviting others to "send me anonymous messages".

The post can be sent to Snapchat users in a person's friends list or can be shared to the wider public.

The person who sees the request can then send an anonymous message on Yolo itself.

If the original sender decides to respond, their reply is in turn posted back to Snapchat.

It's really unusual for an app with little media attention or promotion to make it to the top of the iTunes charts.

It was created by a small French company called Popshow Inc. They had previously made an app that let people post their reactions to funny videos.

Newsround has been unable to reach the company for a comment.

You have to be 12 years old to use Yolo and some children's charities think this is too young.

That's because a lot of previous Q&A services have been used for bullying and abuse.

A spokesperson from US internet safety group Protect Young Eyes told the ´óÏó´«Ã½: "Anonymity has always created a breeding ground for hate and very poor teen decision-making."

The children's charity NSPCC think Snapchat should step in because it has a responsibility to protect children.

"Apps such as Yolo that allow anonymous comments could be easily misused to send abusive or upsetting messages. Snapchat should justify how this app meets their duty of care to children" said the NSPCC's Andy Burrows.

A spokesperson for Snap - the company that owns Snapchat - has told Newsround that Yolo is "not owned by or affiliated with Snap in any way".

According to Snap, Yolo operates on "Snap Kit" and therefore still has to follow Snap's Community Guidelines, even though its not affiliated with Snap.

That means users can report bullying or intimidation on Yolo through Snapchat reporting tools.

The spokesperson said: "When we are notified that a Snapchatter is violating our rules, we promptly investigate and remove the offending content, if appropriate, and may terminate the account."

They also added that the app doesn't appear to be like previous Q&A services because anonymous answers aren't then published publicly - only the person receiving the message sees them.