So, Facebook added their Chat application but still haven't added the promised Jabber support so that you can use it through a standard instant messaging client. (If you haven't ever used one I recommend Pidgin - you can use it to do messaging over Jabber, Google Talk (which uses Jabber), MSN, Yahoo IM, etc, etc, in a unified way without having to install hundreds of programs. There's also a Mac-specific version called Adium.)

Fortunately, Eion Robb (who did the Skype Pidgin plugin) has come to our rescue with his Facebook Chat for Pidgin. Install it and not only can you do chatting through Pidgin, but all your Facebook notifications and friend requests can be managed there too. (Jabber support would be even better because then you could chat between different systems, just like you can email between them, but this'll do for now.)