Why? Not many people remember many peoples' birthdays: I know that because of Facebook I don't need to proactively ask for and store most friends' birthdays because I'll be actively reminded of them.
In reference to OP, it's a matter of thoughtfulness. A Facebook birthday notification happens whether you're thinking of a person or not. At least, if you put it on your calendar, you had to sacrifice some time and energy.
Not to say that you're less thoughtful for using Facebook reminders; it just seems to promote shallow interactions at times.
