App store rankings based on downloads or ratings? http://dd508hmafkqws.cloudfront.net/sites/default/files/B8z3...
Yelp ratings? http://www.avclub.com/tvclub/every-person-here-thinks-about-...
Basically anything with a rating or ranking system where individuals contribute to an aggregate score and the highest scores get more attention, and if you get more attention you get more revenue/exposure/even more attention.
1. Will a person in your target market, when cherry-picking a couple examples they care about, say "Wow, these results are way better than what I would've found myself!"?
2. Are there enough people in your target market to justify the economics of your system?
Answer #2 first, and then put all your effort into making #1 come true.
Google worked because everybody searches the web, and Google was clearly better than rival search engines. Many metric-based businesses fail because not enough people care about the metrics to make enough money; the remainder fail because they're not a noticeable-enough improvement on the status quo.