I'm not a big fan of Facebook, I don't update it too frequently either, but some days ago I found that these days Facebook have released a new app Facebook chat, which is based on its acquisition of Beluga.
I've tested it on Blackberry, on my Android and also on my iPod Touch and it works great on all of them.
I must say that the guys at Facebook did a good job integrating the app with iPhone and Blackberry as it looks like the default message applications on those devices, with blackberry you even have full integration with the message system and with iPhone the notifications work really good.
I've also seen that if you are not active on chats, I mean not actively using the app, the battery and bandwidth consumption is very small, due to the use of MQTT protocol which according to Facebook engineers keeps battery and bandwidth usage low.
MQTT is specifically designed for applications like sending telemetry data to and from space probes, so it is designed to use bandwidth and batteries sparingly.
What I can see is that considering that almost everybody have a Facebook account, and that this is not a Facebook app, but just a stand alone chat app, the future of Whatsup and other similar apps is not clear at the moment. Talking about BBM I think is a different story, BBM is used a lot by the enterprise market because it is secure, we can not say the same about anything in Facebook. But for the teenager and friends group of friends is a great option, as it is cross-platform, works great in each of them and you can also use it on your computer or iPad or Android tablet, something you can't' say about Whatsup, where even if you are in front of a PC, you have to deal with the phone keyboard to chat with your friend. Something I personally dislike a lot.
I encourage you to test it, at Facebook site
And for those on Google Circles, yes, that is an option too, but there is no Blackberry app yet, and the one for Android keeps crashing, so, some more time is needed.