I think it's a bit different, on HN and Reddit it's much less about sharing ones life (there are subreddits for this, but in general, no). It's more about sharing interesting discussion and insight around content.

More than that, it's semi-anonymous, meaning it's not so much the relationship between people it's the relationship between content (or topics). This is a dramatic difference because it's harder to get into the loop of seeing other people happy.

Finally, there aren't push notifications for HN. I actually wrote a chrome extension to enable following discussions and push notifications.. I didn't really think it was appropriate to release because it would lead to more engagement, which is kind of a bad thing (e.g. lead to flame wars).

Anyway, they may not be "that different", except in a few key aspects.