People (in the UK at least) have grown so 'comfortable' with using SMS - I'll admit to it myself, I could easily fire up the Facebook and send a message to any one of my friends, all of whom have smartphones and would undoubtedly receive the message within a minute of it being sent. However, for whatever reason, I'll still just send an SMS.

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.