The Recommendations page on Amazon shows me stuff like the stuff I've bought in the past. They also develop an idea of how often I buy certain things and automatically recommend them when it's possible the last one they sold me is exhausted (vitamins) or needs replacement (toothbrushes).
I was curious to see what 3TB disks sell for now, so I did a search. Now, for the next few weeks I'll see disks and other computer accessories in Recommendation. But over time it will drop off, until I remember to look again.
I spend a lot of time thinking about news, and listening to people's dreams for how a news system works. If I were to distill down everyone's wishes to a single expression it would be this: Find me stuff I'm interested in.
I have a feed of things I link to, which then flows to River2 subscribers, and through TwitterFeed, to people who follow me on Twitter. Every one of those links is potentially telling my news system that this is something I'm interested in.
When I push a link out not only am I sharing the link with people who follow me, and bookmarking it for future reference -- I could also be informing a news engine of my interests. So then what comes back to me, on my River page, are news items that relate to the things I'm interested in.