If you use iChat to communicate with others you can use it to talk to friends on Facebook too!

Reposted from Mac OS X  Hints:

• Find your Facebook username by going to http://www.facebook.com/your_user/.
• Next, Open iChat, then select iChat » Preferences and click on the Accounts tab.
• Click on the + (plus) sign to add a new account, with these settings:
• Account Type is Jabber Account
• Account name is your_user@chat.facebook.com, and enter your password
• Click the drop-down arrow to reveal Server options. Enter chat.facebook.com as the server name.
• Enter 5222 as the port and click Done.
• Click Done again, and you are good to go.