I have to second all the other answers. I guess the term "social network" carries a lot of (crappy) connotations with it, but if it lives up to the promise, connecting like-minded tech people to hang out and share knowledge: Sure.

I just have some difficulties to imagine what it would need to look like to a) don't end up copying FB and b) not copying HN itself

So I'm on the edge on this one. FB et all don't interest me, this is interesting because of the (controlled, on-topic, interesting) contributions. How are you planning to build a social network with the same merits? Are you going to build a new, different Identi.ca? (which, imo, is a failure in terms of adoption, and they mostly targeted the "tech" demographic)