That's probably because Facebook is really just a way to satisfy narcissistic needs. I've seen to many people writing their diaries on FB, open for everyone to read, trying to grab attention. In a way, a lot of publication platforms are like that but FB has become the worst kind.

Also, the technical nature of HN attracts a different kind of users, who are more open to deep discussion on interesting topics. That's just a part of hacker personality I guess.

For socializing with an audience, a platform like HN is a lot better than FB. For personal communication, Email is still the very best thing we came up with, even though we can video chat, make VoIP calls or instant message. To me, Emails are an entirely different form of communicating, just as letters were back in the old days. Almost every other form of telecommunication tries to mimic real life conversations in one way or another, Email doesn't. I like that.

Bottom line: There's nothing except for advertising and handling sensitive information carelessly that Facebook is truly good at these days. But that's - as always - probably just an opinion.