I get that, and should any concerns arise, we would react swiftly.
phpBB showed birthdays to guests for a long time, if I am not mistaken. I think it was changed with 3.1, was it? During all those years no member of ours ever mentioned they wouldn't want other than registered members to see their birthday.
The difference between Facebook and phpBB boards is that most people on Facebook use their actual and full name, whereas most people on phpBB boards -- or so I assume -- use a nick.
"Before I speak, I have something important to say."
- Groucho Marx