Yesterday evening a posting on the Facebook Developer Blog announced support for the XMPP protocol (also known as "Jabber") within Facebook chat. This addition means that users of Facebook are able to connect with their friends instantly using a client such as Pidgin or Adium, without having to load the full Facebook page. Facebook chat currently sees over two billion messages being sent per day, and with support from desktop clients via an open source protocol, this is only going to grow.
Whilst most Jabber implementations support server-to-server connections, it appears that Facebook has not included this feature, and there are no indications as to whether it will be added in the future. Server-to-server communications allow contacts to communicate cross-domain, for example, were it to be included at a later date, it would be possible to communicate with Google Talk users via your Facebook chat account.
While a number of Instant Messaging clients have added support for Facebook chat in the past, these implementations have been prone to bugs, by enabling users to connect via a tried and tested protocol, Facebook have improved the end-user experience, and made it a lot easier to keep in touch with your Facebook friends. Facebook have also launched a page with a simple guide to setup popular instant messaging applications.