If you want to modify SNS contacts, then you have to create a dummy contact with the same name. You do not have to put any more information in the dummy contact. In fact, you don't even have to use the last name. It just has to have something in it that comes close. Then you save it.
After you do that, then you go to your contacts and open up the one you want to modify, and you join it with the dummy one that you just created. Click on Menu at the bottom of your phone. Then click on join contact. This gives you the option to merge your original contact with the dummy one that you just created.
Once they are merged together, then you can edit your contact.