They're not thinking of you because Facebook had to remind them.
Try making your birthday private in Facebook one year and see how many happy birthday messages you get when people aren't prodded by Facebook to care. It's a lot less.
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.
Just a few more poorly measured statistics from me:
I have 1200+ friends, have about 10-15 birthdays a week. Never post on their walls, don't have my birthday listed. Get maybe 10-15 wishes a year on my wall. Text/call about 10-15 people a year (not including family). Before I removed my birthday, I had about 700 friends and would get around 100 wall posts.
Going to try an experiment where I make a random day my birthday, delete all the posts that catch on to the fake birthday, and then change it to a month later. Will post results.
Yeah, I have seen people do this, definitely will get a lot of messages still. Facebook limits the amount of times you can change your birthday, likely because of this.
That doesn't mean that people don't care. I certainly wouldn't able to remember the birthday of all my closest friends, but I do care and am glad to be reminded. On the other hand, there are of course a ton of bday wishes from people who probably don't really care, but is that really a bad thing?
Yeah, I think you can be reasonably good friends with someone and not remember their birthday. Unless you are really focusing on it you are going to have a limited bandwidth for remembering birthdays and likely have more people you genuinely care about sending your best wishes than you can remember.
You were pro-active and took steps to remember the birthdays by sticking them on your calendar. It's different from clicking on a link because Facebook remembered someone's birthday.
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.
Try making your birthday private in Facebook one year and see how many happy birthday messages you get when people aren't prodded by Facebook to care. It's a lot less.