I am going to write with an assumption that most of you have droids. If you do, I think you should do a system update (available under Settings). After you update your phone, you will see some changes in the droid interface, particularly the android market.
Let us say you try to edit your facebook contact. For instance you want to save this cutie's phone number after you had flirted with her on FB. Sometimes (or most of the times for me) you will see a message indicating that "Facebook contact is not editable". Bummer! Not to worry, there's a hack for this until another system update solves the problem.
Select your FB contact. Click on EDIT from the menu. Now you should see the message saying your contact is not editable because it is an FB contact. Now, click on the MENU again and click ADD CONTACT. Enter the first and last names of your contact again (spelling should match your existing contact). Enter the phone number, then SAVE. Boom! You should see that the phone number is picked up and merged into the existing FB contact.
Option 2 (I highly doubt this will happen):
This happens if option 1 still does not work and now you have duplicates. Select your FB contact. Select MENU, then EDIT CONTACT. Now select the MENU again and choose JOIN. Then select your new contact.