The app developer, just based on your description, is not responsible for what its various members do. It is no more responsible for that than, say, the phone company is for similar conversations like this that occur over the phone. In both cases they just provide the medium for people to communicate. It is the adults that go after the kids that are the problem, along with parents who do not watch what their kids do on their computers and phones. Too many parents seem to have no idea just how much trouble their kids can encounter online and don’t take the effort needed to monitor what their kids do. And as you noted, there are lots of chat programs and computer games with chat features that kids use that also draw adults looking to chat with them.