Next, open Message on your Mac, go to the ‘Accounts’ tab in its preferences and add a new Jabber account. Under ‘Account Name’, add your Facebook username followed by ‘@chat.facebook.com’ (without the quotes). Enter you Facebook account’s password under the ‘Password’ field, click ‘Done’, and you’re really done!
Open Messages and wait until your Jabber status changes to ‘Available’. In the ‘To’ field, start typing a friend’s name and it will pull a list of friends from Facebook, searching them as you type. Messages will also show a friend’s online status. Messages you send and receive will be saved by Messages, while you can still access them on Facebook itself as well. For those conversations that you’d prefer not to keep a record of, here’s how you can delete your Messages history in OS X Mountain Lion.
For those who haven’t upgraded to Mountain Lion, this trick will work in the iChat app as well, since it can also connect over Jabber. If you’ve upgraded to Mountain Lion but haven’t connected your Facebook account, you need not worry because this method with Messages works independently of it. Facebook will detect when you are using another chat client, and its web alerts i.e. those annoying sounds that make me think my battery is about to die will be automatically disabled. You will be able to send messages to both online and offline friends, but Messages will not be able to mimic the ‘Seen’ feature that tells you a message has been read. If you are using a separate chat client merely for Facebook chat, this method can prove to be one very good alternative to it.