Of my friends who are friends but not best friends, I only know one of their birthdays. I only know that because her Xbox Live gamertag has her birth month and day in it (and she was born one year and one month before me, kind of interesting I guess).

Birthdays are a thing that are hard to remember. People used to mark them down on a calendar or just wait for the person to remind them. Both of those functions have been replaced with Facebook now.