Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Use iChat to Communicate with your Facebook Friends
Facebook chat can now be synced with your Mac’s iChat. Here’s a short guideline on how to do this.
1. Open iChat. Click Preferences, Accounts tab, and add a new account by clicking the plus sign.
2. From the Account Type drop-down list, select Jabber Account.
3. In the Account Name field, enter your Facebook username followed by “@chat.facebook.com”.
4. Expand the Server Options list.
5. Fill in the following data:
• Server field: chat.facebook.com
• Port field: 5222
• Use SSL checkbox: cleared
6. Click Done.
Now the usernames of your Facebook friends will appear in the iChat window along with the names of your iChat buddies.