I have a friend who once spent an entire month logging in every morning and setting his birthday to the current day. And yes, he did get many, many erroneous birthday wishes from people who didn't notice anything was up.
As for me, I don't list my birthday to anyone. I don't like receiving those post floods. (In fact, really, the only information I list is contact info, since I only use FB for keeping my contacts up to date.)
That's a brilliant experiment. Proves my point.
But, not keeping ones birthday listed prevents the true contacts from reaching out to you. I've people who call me up on my birthday who although reminded by facebook at least called me up. Granted, it's not sincere, but at least we have fun catching up.
I reached a similar conclusion a while back and actually ended up writing an app that would automatically wish friends happy birthday when I was trying to get to grips with the facebook api:
http://blizzardsystems.co.uk/dr/node/44
Ha ha ... That's a great tutorial and a brilliant idea. It could prove to be quite useful. But, somehow I can't bring about myself to using it - seems like loosing my last bit of humanity.
As for me, I don't list my birthday to anyone. I don't like receiving those post floods. (In fact, really, the only information I list is contact info, since I only use FB for keeping my contacts up to date.)