The primary issue I have with the Facebook messenger platform is that there's no distribution/promotion channels. In the iOS app store, new apps appear in the "New" section and you can get some new users that way. In the old days of Facebook, there was a similar system. With chat bots, I don't see how to promote a bot once it's completed. There's no bot store. Essentially, the task becomes asking users to have both Facebook + Messenger, and to install the app by sending a message. That sounds like a lot of friction.