Well, if you block them they can't see any of your activity, and I would assume that would include you poking them...
1) If they see it before you block them, then it wouldn't matter and
2) If they have their notifications set up to notify them of pokes, it wouldn't matter if you block them before they log in to FaceBook, because the instant you 'poked' them, a notification would be sent out and cannot be revoked/deleted by FaceBook no matter any changes you make.
it means either your friend's account is deactivated, they arent on the new facebook messaging system yet, or, Facebook is broken..again..
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