"By this point, your Paparazzi app looks up real users on Flickr, finds and lists their photos and lets you view them up close by zooming and panning. It also plots these photos on a map and lets you browser photos by location."
How about this one : Faceless. Social sharing of pictures of your possessions and favorite things that are guaranteed by face recognition not to contain people in them. Find and meet new people by interest, taste.
Hacker News is bad at a persistent conversation of this type though, it will drop off the front page in a day and no one will add to it further. Something like this benefits from longer conversation though as it is a day to day and ever changing discussion.
So we'll do it again next year. It works for every other topic.
If the problem with a point-in-time conversation of this kind is that it goes stale, the problem with wiki-style "persistent conversations" is that they also go stale, only piecemeal, like Swiss cheese. You visit the wiki and have no idea which bits are new, and which are old, and whether or not two things that happen to be next to each other were written in the same year or with any reference to each other.
If the wiki were periodically edited, in toto, by someone who cared, this problem could be avoided. But most are not.
People are happy to have the same conversations again, only different. Conversing on HN is not drudgery to be automated away. We like rituals. We like the classics. And new people like going through the classic exercises that their predecessors did.