I would prefer showing the full name.
Usernames can be anything under the sun,eg. 'liferocks', 'rockingkool' 'mrsupersmart' etc. whereas actual names would be far more sensible (or atleast there is a higher probability of people giving a sensible full name as compared to a username).
As you said it is to denote who all have become your friends, it would be more sensible if the user sees that John Doe is his friend instead of 'rockingkool'.
If needed you may also show username with less emphasis(parenthize, smaller font, subtle color etc) next the Full name.
Eg. John Doe (rockingkool).
If you have a profile picture then estate should not be a problem around the picture to accommodate both of them. Then again, as I said, only if its required.
Edit: Just to add further clarity, my approach with the full name stands for the people in my friend list.