I have the SDK 3.1 working for iOS 6 for posting status updates and I have the facebook user id of the person who I want to post on their wall.
I just want it to say "hi." to just one person's wall. I can target their feed with "/[id]/feed" but then I have to supply a graph object? if I use startForPostWithGraphPath() anyways.
I figured it out!
First of course implement FBFriendPickerDelegate per here, unless you already have their friend ID somehow. http://developers.facebook.com/docs/howtos/filter-devices-friend-selector-using-ios-sdk/
Then the important part, how to post to friend's wall: