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how is it much different from the ranking engines netflix and amazon use?

In those cases, users purely receive recommendations for media/products. In this case, users receive recommendations for other users to follow as well.

You can calculate the "weight" of the movie for a given user once. The number of movies is very low, compared to the number of stories on reddit.

Whatever that guy is proposing is an insanely complex dynamic system.

I'm not saying it's not doable, I'm just saying it's very expensive.

It might be less complicated then you'd expect (at least one way of doing it): http://en.wikipedia.org/wiki/Collaborative_filtering

The core idea is linear regression.

Here's Google's service: http://code.google.com/apis/predict/. Here's a commercial API service: http://www.directededge.com/.

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