Looks like Facebook silently introduced encryption for chats in XMPP/Jabber clients (Pidgin, Adium, etc.): Its servers now support the use of TLS as defined in RFC 3920.
Facebook’s FAQs (1, 2) have not been updated accordingly yet.
Software Engineer and Consultant
If you are using Firefox with the Adblock Plus add-on, it is quite easy to stop Facebook from tracking you on third-party sites via its Facebook Connect program or by other means. Just add these four rules to Adblock Plus:
||facebook.com$third-party
||facebook.net$domain=~facebook.com,third-party
||fbcdn.com$domain=~facebook.com,third-party
||fbcdn.net$domain=~facebook.com,third-party
Instead of adding the rules manually, you can subscribe to this Adblock Plus filter subscription: Facebook Connect Opt-Out.