Facebook is preparing the launch of their XMPP connection interface which will allow you to use Facebook Chat through iChat and variety of other clients.
According to process one, this is big news for XMPP because it solidifies the protocol as the standard for Instant Messaging. With Facebook adding ~300 million users to the many Google Talk, Yandex, Live Journal and other deployments, XMPP could become the most largely used instant messaging protocol in the world.
Facebook announced their decision to support XMPP in May of 2008. The IMtrends bot has discovered that the XMPP software stack has been deployed on chat.facebook.com.
We are proud that this XMPP support is built on a customized ejabberd XMPP server as announced several times by Facebook development team, including one of their latest conferences in Edinburgh in September 2009: Functional Programming at Facebook by Chris Piro and Eugene Letuchy.
The missing piece is currently what login and password to use to connect, but I am sure it will be announced by Facebook in due time.