I come into it from a slightly different perspective. Even if I never hear from some of my FB friends regularly, I still appreciate even the generic 'Happy Birthday!' message from them. Why? Because they had to put forth some effort to do that. Sure, they were prompted, but they still had to click on the prompt and type something out. Not a big deal, but they still had to make the effort. And to put in in *my* perspective, I have several somewhat close relatives who will not even do that much to acknowledge my birthday, and I know they're on FB pretty regularly. So when a very casual FB friend can take that minute to type out a HB message and that somewhat close family member doesn't bother, well, that makes me have a lot more favorable thoughts toward the casual FB friends and less favorable thoughts toward the close relatives.
And for the situation in the OP, I think the birthday boy needs a hobby if he went to the trouble of contacting the OP about her HB message.