In any social media design you have to take well known power laws into account. You may consider HN currently to be a highly engaging site but you have to consider why that is: a huge amount of eye balls is watching a tiny set of content, a list of a few dozens links. Next, if as little as 1% joins in discussion, the result is threads with substance. Great outcome, but you're probably ignoring that 99% didn't engage.…
Most social networks make posting (or whatever specific terminology is being used) equivalent to talking at someone. The correct model would foster conversation and thus impact engagement. Small social groups discuss topics. Offering a topic for conversation implicitly earns a response. Rarely do groups of friends get together to declare simple and unrelated one-liners. An "ideal" social network might be more related…
Everyone wants to design an online social network as if there is all this spirited high brow debate going on at the local Starbucks when in reality it is a bunch of people shoulder to shoulder ignoring each other.
If people really wanted interaction it would be trivial in the real world. Online social networks let people indulge in their want to talk AT strangers. This is absolutely a feature and not a bug of the software.
Even within myself, the idea of a 19th century style salon sounds really cool but I would never show up to one physically. An online salon with more than a handful of people would be cool but it would break down quickly as it scales up past 20 or so participants until it basically approaches Twitter.