I think you are missing the mini-slideshare aspect to voice and text/image sharing. Just looking at the app website, users can work as a team and use voice for annotation, which is pretty huge vs. text/images only. Plus, any voice elements seem to be treated as part of a larger conversation element that can be resumed or edited at will. Do Skype or Hangouts have these features? I know I can leave people messages like using an answering service, but that's not even close to the same thing.