All iOS users in the US can now make calls through Facebook Messenger.
In the last few months I've started to use Facebook Chat a lot more through the Facebook Messenger app on my iPhone. I've found it to be a lot more intelligent than using iMessage. For example, when I'm at work I can use Facebook Chat through the website and continue where I left off on my phone; complete with conversation history. This is a great example of the continuous client. Also, it seems like it detects where you are. If you are sending messages through the website it seems to not push a notification to your phone. But if you don't answer for a few minutes it then pushes the notification.
iMessage is pretty good at keeping my Mac and my iPhone in sync with the conversation but if one of the messages fails to send through iMessage and goes as a text instead then it won't show up on the Mac. It also just seems to spam every single one of your devices when you get a message.
Now that it seems Facebook are allowing you to call people through the Messenger app (albeit only through Wi-Fi) it makes the service even better. Also, I think this is a shot across the bow for Skype. I find the Skype app for iOS a bit clunky as you need to set it to always be running in the background to receive calls without the app open. Facebook Messenger seems to work purely on push notifications so it should be easier on the battery.