Is there a Facebook messenger app
I don't know if you're still looking, but here's your answer. It's not an official Facebook app, but I use it every day and I love it.
Here is the link to the site I used to download it
General way
So what about Empathy
?
Usually empathy is installed in Ubuntu when it comes. But if no go ahead install it from here. Or just click button
Note you can't write to offline person. But I think if you will have conversation already opened you can.
Alternative way
So here is alternative way: integrating facebook chat with firefox, read more here
And install plugin from here