I’m not entirely sure I agree. BBM for starters started out in work communities where everyone was rolled out on a similar device/network and grew that way.
Secondly I’m not sure iMessage is as simple to setup as Apple would have you believe. Out of all my friends on iOS I’ve managed to connect with one and he’s what you’d call a tech enthusiast. I’m not convinced personal email addresses are the known entity any more with the introduction of Facebook. I think you are more likely to have someone’s mobile number than their email address (which happens to be tied to iMessage/Facetime).
And lastly it isn’t cross platform. This is why things like Viber, eBuddy XMS, Facebook Messanger and Google+ Messanger have an edge, particularly in the case of Viber and XMS as they use your mobile number and automatically discover contacts in your address book.