I just read that Facebook chat is now based on Jabber/XMPP. What does this mean? Basically, any of the instant messaging clients that can talk to sites such as Google Talk such as iChat, Adium, or pidgin can be used to connect to Facebook.
First, you need to make sure you have an account setup on Facebook — not just able to login to Facebook but an actual, short name. Go to “Account” menu:
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And then you can set your “Username”:
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Then you can setup your IM client to connect using the “Jabber” (or XMPP) protocol with your Jabber ID as the above username@chat.facebook.com. This example is for Adium but other clients are similar:
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You probably don’t need to change the port to 5222 but you may need to make sure SSL/TLS is disabled.
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After that, it should connect to Facebook. You don’t have to have Facebook open in a browser.