Facebook's chat service is powered by the Jabber/XMPP open source chat protocol. Many networks (such as Google Chat) rely on Jabber, and for this reason chat clients like iChat and Adium allow for the setup of a generic Jabber account to facilitate the widespread use of Jabber.
The process is simple. First, create a new generic Jabber account pointed at Facebook's Jabber server. The username to use is your Facebook username (which you can find on the Account Settings page) and the password is the same as your Facebook login password. Next, point the server to "chat.facebook.com," configure the port to 5222 and un-check the box for "Use SSL."
After it's all said and done, you should be able to communicate with your Facebook friends using iChat without being logged in to Facebook with a browser.
[via Cult of Mac & 9to5Mac]