Cross platform FaceTime / iMessage will be THE killer app.
Skype could've been it but Microsoft destroyed it.
Surprised Google hasn't been able to do it yet.
Hope WhatsApp doesn't win this space (shudder).
The clock is ticking.
Meanwhile on Windows Phone, it was discontinued on WP7 [2], while WP8 had native 'iMessage'-equivalent apps backended by Skype, but also a downloadable Skype app. If you installed both, you got double notifications. Was it supposed to be the platform-native messenger? On the phone, the answer was 'yes', but on the desktop it was 'no', or 'not yet'.
Desktop Windows 8 eventually also got a Metro app, but then it was killed [3]. Now they're about to release another one [4].
[1] http://www.theverge.com/2013/2/15/3991370/windows-live-messe...
[2] http://www.windowscentral.com/microsoft-discontinues-skype-w...
[3] http://arstechnica.com/information-technology/2015/06/micros...
[4] http://blogs.skype.com/2016/03/24/skype-universal-windows-pl...
It's owned by FaceBook.