People already just like photos like that. Or say simply "happy birthday" to everyone when Facebook tells them it's time. It's a small step from this state to a slightly extended set of canned responses to pick from.
The "Happy birthday" messages were nice the first time (circa "thefacebook"), but the more I thought about it the more depressing and creepy it became. I hid my birthday, then closed my wall, and then left facebook entirely. Now it's only sincere messages from a close few, which feels like much more.
definitely agree, on the other hand it's funny to see people who put fake birthdays on facebook actually getting the "happy birthday!" messages on the fake date