2 experts have answered
Any messaging app that encourages or gives you options to share your messages or user name on social media will be a problem. Whatsapp wants you to share your user name on Twitter and Facebook. This is why the other messanger Kik is having problems with kids being friended by adults. Messaging only apps like iChat. Personally I only like messaging apps that don't link with other platforms. SMS style apps, or Skype.
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Leonie Smith has got the right idea about this topic. Always remove particular app names from discussions like this. Rather than recommend one app over another, judge the app or technology by how connected your kid can get while using it, as Leonie says. If you can't see who your child is communicating with, he/she doesn't need to be using it.