Actually, WhatsApp service that targets Android and iOS. And they made adding contacts super easy. No ICQ number, no XMPP handle, no username. iMessage is Apple-only, Google Talk has a slightly different concept and more complicated.

any messaging app included by default in Android will have users migrating in droves

Yeah, that's why Google+ replaced Facebook and everyone is using Google Talk. Nearly every browser comes with Google as search engine or homepage and it didn't worked.