I mainly just talk about things I'm genuinely interested in. That makes me naturally more conversational in my posts, comments, and replies. When someone replies to me, I want to acknowledge their contribution, so I try to at least say something relevant to their comment. Hopefully it's something that can get another reply out of them or someone else, but you at least want them to feel their comment was appreciated more than just an upvote or a one word response. I think that encourages them and others to comment again, as even if you can't add much to that current thread, you show you're there and open to conversation and that you are willing to engage if they speak up first. I will reply to people that comment on my old posts because I'm still interested in the original topic (I posted it after all!) but if I'm reading someone else's post and it's much over 24 hours old, I'm usually assuming they've moved on unless others are still interacting with it as well.