FacebookChat is a plugin for Pidgin that allows connecting to Facebook chat from Pidgin instant-messaging application. However, as of recently, some changes at Facebook have caused this plugin to be unable to participate in chat sessions as a transmitter. It would happily receive messages but could not send any. The message that would pop up was
"you are not permitted to do that"
Looking for a solution, I've just discovered that Facebook has enabled XMPP- (Jabber-) style chat. At the page below you can find instructions on how to set your chat client to connect to Facebook chat directly.