It won't let you edit anything that was imported from Facebook. What I did was add a separate contact with the other phone number/info and "merged" the Facebook contact to the new one.
Now it shows me two halves of the contact, the top half is editable (name, phone number, etc.) but the bottom Facebook half is not.
NOTE: The only thing that still sucks is that the name that I put for my contacts is overridden by the name imported from Facebook. A little annoying since I know some of my friends by their middle names/nicknames and I struggle looking through contacts for them because they have their full name listed in Facebook.