Messaging is already implemented; it's just only enabled for conversations between us and YC founders.
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.
The "friending" should just be an asymmetric following system, like Tumblr of Twitter. I want to see a stream of stories that my "friends" have upmodded, with a stream of threads too. I find myself going to my threads, and editing the url to see other people I track, all the time.
I just found myself wanting this morning when I was reading through the comment threads of a friend of mine. I really wished that I had seen them at the time they were posted instead of 3 months after the fact.
I don't think this would be a particularly hard thing to build as a weekend hack...
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.