I am big fan of pidgin, the multi protocol free and open source messenger. It supports Yahoo, Gtalk, MSN, AOL, etc… on one single app. And has an huge list of useful plugins.
When facebook allowed third party clients to communicate with their facebook messenger infrastructure, pidgin immediately added support for it . You can try configuring facebook account on pidgin with the help of this page
However the default support inbuilt in pidgin doesn’t seem to work anymore on Pidgin 2.7.8 and later versions.
Luckily there is a opensource Facebook chat plugin for Pidgin. Download the appropriate installer for your OS from here and install it. Restart the Pidgin. Before configuring the account you need a facebook username. This is different from having a facebook account. Though you might be using the facebook account for a long time it is possible that you might not have registered a facebook username. Register the username here before you proceed.
Ignore the Facebook(xmpp) which is the inbuilt facebook module which is not working for many.