I don't know... We think something like, "Hey, it's so-and-so's birthday today. I wonder how s/he is." If we were particularly close to them for enough time it can, in some ways (and yet not other ways) seem kind of strange not to even consider calling the person. Then again, with moving on and/or going in different directions being what they are, it would also be strange to think about trying to contact that person as well.
And yet, each year on that person's birthday we do tend to notice that date. It's just kind of strange; that's all.