This is a common problem with all internet websites. They want to know my date of birth. Because they want to ensure that either I am above 18 years old or that I need to be wished by other folks on my birthday.
How can I be sure that their database is not compromised? Just because I get a lengthy privacy safeguard letter or url from facebook, orkut and other websites, it does not mean that I can feel SAFE.
Look at a detailed look at psychological, social and privacy related issues with Online Social Networking sites at my blog entry.
Basically, what Facebook is trying to do is comply with COPPA (Children's Online Privacy Protection Act) to try and find if you are above the age of 13 years of age and in addition tell your friends about your birthday. By default, the birthday is public in your profile (which is another screwed up default).
I think the following would have sufficed and been better:
a) "Is your age 13 years or above?"
b) Please give us your date and month of birth.
DO NOT FORGET TO TURN OFF DATE OF BIRTH VISIBILITY IN YOUR PROFILE.
Dangers of Facebook in a corporate environment: http://anil-identity.blogspot.com/2008/11/facebook-and-corporate-users.html