I'm no less disturbed by this than anyone else, but you do not truly own your Facebook account in the way that you would own a PO box.
Your data belongs to Facebook, not to you. Sad, but true.
Edit: (submitted too soon) I'm sure there's something legally dubious about requesting private keys in this way, but the PO box example was a clear reminder to me about how we wrongly think of our data on private services as "ours".