I think there is a fine balance between being "strung along" and gradual engagement. For instance, if I were to use an app to share photos (a la flickr), and I

I think there is a fine balance between being "strung along" and gradual engagement. For instance, if I were to use an app to share photos (a la flickr), and I was invited to upload a bunch of photos then be presented with "Ok, now you've got to sign up RIGHT NOW or we'll delete everything you just did" - that's sorta missing the point. That's less gradual engagement and more impending doom. The implementation of the chatbot above seems to be more along the right lines, though. The holy grail perhaps would be allowing the user to fully engage with the service without even realising it.