If we could send messages privately/directly to each other through HN that's when it'd become 90% of what a social network is IMHO. Friending would take it to 95%.
One reason I haven't turned it on is that I don't want all the responsibilities that come with running a message service. But if a lot of people want this, I'd consider turning it on for everyone.
I don't think this would be a particularly hard thing to build as a weekend hack...