That's not accurate analogy. Meeting someone once makes you a lot closer than a lot of these facebook "friends" are. Even calling someone up once a year just to say happy birthday shows a level of familiarity and care. Just the simple action of a phone call to an individual in your example shows that you care. In your analogy, you couldn't have known the person was dead until you called. But in this FB instance, they had every opportunity to know he was dead because all of his information was right there for them to see, and all it took was to bother with the simple action of literally one extra mouse click. They did not even manage this.
Having Facebook tell you it's someone's birthday, and clicking it and typing "happy birthday" doesn't prove any level of care, especially as we have established that they didn't even go to their wall to see the obvious messages about someone's death, proving it was nothing more than going through the motions.
When you have 4000+ facebook friends, chances are a lot of these people have never even met, nor called, nor even said "hi" to each other in a PM. It's just padding a number on a list for the sake of networking and self advertising.
I'm not saying that if you say happy birthday to someone on FB, that you don't care, or they don't appreciate it, I'm merely saying that cases like this prove clearly that these particular people didn't care in the slightest, and that it shows that a birthday message on a social networking site doesn't necessarily mean anything. Ironically, I'd say it just highlighted how impersonal the action was, and how little they actually cared.
FB is entirely what you make of it. For people like you and me, we use it to become closer to people and keep in touch. For some people, it's just become a routine focusing on the networking rather than the socializing.