I hide my birthday on Facebook anyways, but I also locked down my wall so people can't write on it. That cut down tremendously on the amount of account-hijacking spam that was getting posted to it from my Facebook friends. I still got 3 Facebook messages about my birthday and my wife posted it on her wall for another 20+ birthday comments. I'm tired of it. I only wish people a happy birthday on Facebook if I either know them really well, they're hitting a milestone like 16 or 50, or if they're really sick.