I guess I don't pay that much attention to usernames. I've been browsing HN since 2011 and I recognized 5 usernames after scrolling (fairly fast) through the whole list.
I’ve noticed that same thing. On other sites, I get into repeated and ongoing conversations. Maybe not like making friends exactly, but a welcome extra level of connection sometimes. Doesn’t feel like it happens here, at least not for me.
I wonder if simply bolding the username, or having it on its own line, or some other subtle emphasis would be enough to lift usernames in our awareness.