Facebook chat via Empathy, unlike Pidgin, is actually very easy to set up.
First, go to Edit > Accounts, then click Add… and select Jabber as the protocol.
Fill in the screen with your Facebook username (NOT your email address) in firstname.lastname@example.org format, and optionally check the Remember password checkbox and fill in your password.
That’s it, you should be all set! If you get a certificate error when signing in, see this article for how to resolve it.
Questions, comments, and feedback are welcome and appreciated!