I think email notifications should be sent to all user's friends when it's a user's birthday. I just checked the email notifications settings and saw that emails are not sent out to friends of a user if it's their birthday.
If this function is active, more than 100 users that are my friends would visit my site today, because, today is my birthday. I felt sad that all birthday greetings were posted on my Facebook page when almost all these guys are also users on my website.