Facebook Messenger, SMS and iMessage
brooksreview.net
brooksreview.net
Apple have realised that the only way of replacing this (and eventually killing SMS) is to provide a messaging service which is inherently invisible or 'passive' from a UX point of view. Yes, it is tied into iOS, but the critical thing to consider is that users are required to put in zero effort to send an iMessage.
With this in mind, I don't see how Facebook Messenger is any different to the likes of [insert free web messaging service here], admittedly with a lot more users.
Whereas with Facebook, just about everyone has an account.
The killer feature really is group messaging on-the-go with all of your friends, regardless of their phone. As it stands right now, I have a feeling Facebook are the only ones really going for this.