I recently noticed that only a couple of my contacts' birthdays show up on my calendar on OS X. For instance, I have four friends with birthdays today, but only one is on my calendar. I tried deleting and re-adding my Facebook account but notice no difference.
I'm getting the exact same behavior on my iPhone.
Maybe this isn't a Mac issue. Did Facebook change the way its sync works?
Best Answer
Facebook has an option for friends to Opt OUT of their birthdays being displayed to other people. So if I had you on FaceBook and I turned the option off to display to you what my Birthday is, the it won't sync to your Calendar.