He or she won't ever notice this. After three posts he or she was last seen in May last year and is not likely to ever see this.
Happy birthday to him or her anyway of course (I don't begrudge anyone I don't even know for a happy birthday), but if those we don't know at all or who never posted (this too happened) get just the same attention as those we really know and care about on a more personal level, the question remains how much worth the congratulations are anyway :unsure: