Facebook chat is the primary real time chat system that I use these days and has replaced MSN for me and my friends. I recently installed the Messenger app on my iPhone and it certainly has the potential to kill off whatsapp and similar apps due two reasons: all my contacts are already available, and that I can use either my laptop or phone, depending on whichever is available to me when I need to communicate.
Unfortunately I doubt that this potential will be realised. Why? Because I haven't seen Facebook advertise the app AT ALL. (I heard about the app through HN.) None of my friends are aware the app exists and it's such a shame. It wouldn't be hard for Facebook to push their chat service as an alternative to txt/bbm/whatsapp, they already have an ad network in place that they can use for free.
I really hope to see Facebook push their messaging service as a way to communicate not just when you are browsing facebook.com but instead as an alternative to text-based communication methods on phones.