I've reached the number of friends on Facebook where at least ONE person is having a birthday every day.
I probably only post 10-15 "happy birthday!" posts a year. So every day I check to see who's birthday it is. If it's someone I'm close to, then I post something.
If Facebook didn't have them displayed, I would probably have those 15 birthdays written down on a real world calendar, and I'd just check that.