In my facebook experience, people on my friendlist only see what they want to see. This isn't to suggest that no one wants to know it was your birthday or that they don't wish you well, more that most people possibly didn't pay attention to the notification when they received it (from memory, facebook sends an email to you saying someone's birthday is coming up, and it also states it, in small text, on the top-right box of the facebook wall screen).
Personally, while there are a number of people on my friendlist who I care for, a birthday isn't really something I go out of my way to acknowledge for the following reasons:
1. Some people dislike being reminded they're getting older
Actually, that's the main reason I can think of right now: when an online friend makes your PC remind you that you're then officially 1 one year closer to being past your use by date.
Do you come across to others in a particular way? Are you generally seen as being a 'happy' person, or someone who communicates with alot of people you know and takes an interest in their life by asking them questions and seeing how they are, or are you more someone who tends to tell people more what you think, or your feelings on something, while waiting for their acknowledgement?
My point being, do you say happy birthday to these people (and I'll assume the number of actual friends you regard yourself as having is around 2-3 doz of those 500 on your list? - ie people you talk to all the time, as opposed to people who are just there for the heck of it)?
Don't forget that facebook also has the 'hide this person from your newsfeed' option, so many of those 500 probably have no idea your birthday has been and gone. Unsubscribing to someone isn't necessarily an insult. I have alot of disabled friends, for example, most of whom live in the US and are experiencing very tough financial times where they've lost jobs, or their homes or something like that. If you're having a truly bad time in life (as some on your friendlist may be) would you honestly take the time to go wish someone well for their birthday if you knew you didn't have to?
I think many/most/the vast majority probably wouldn't, except for maybe your family.
I suggest you stop stressing/caring about this using the following mindset:
1. Do you really want 500 people reminding you that you're a year closer to dying of old age?
2. If you feel willing to think about birthdays, but everyone on your friendlist doesn't, doesn't that make you a more positive minded person than them (possibly)?
Failing that,
3. Make sure you acknowledge every single birthday of everyone on your list when their day comes around. Many will possibly unfriend you because they've unsubscribed to your updates and possibly forgotten you're even on their friendlist, but some will automatically then ask themselves, 'That was nice of
to do for me, when is birthday again?' then they see it's passed by and they've missed it but you didn't miss theirs so they want to make it up to you and they speak to you more, making your friendship with them better.
Course, you could always take my approach, which certainly helps me sleep alright at night,
4. Birthday? We still celebrate those? (ie treat the concept like it doesn't exist)