Facebook made some changes in his API, so any account trying to access from Pidgin Instant Messenger was unsuccessful, until now, thanks to James Geboski and his new plugin for (L)Ubuntu, Debian and OpenSuse. To install put these five command lines in your terminal:
sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/jgeboski/xUbuntu_$(lsb_release -rs)/ /' >> /etc/apt/sources.list.d/jgeboski.list" c
cd /tmp && wget http://download.opensuse.org/repositories/home:/jgeboski/xUbuntu_$(lsb_release -rs)/Release.key
sudo apt-key add - < Release.key
sudo apt-get update
sudo apt-get install purple-facebook
Now open Pidgin, create a new Facebook account (important, not a Facebook XMPP one). This plugin will keep up to date, as the developer will upgrade it. Maybe further versions of Lubuntu or Pidgin will ship this by default. Enjoy!