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In the case of tweets, the key is to identify "reputed followers," says Amit Singhal, a Google Fellow, who led development of real-time search. (Twitterers "follow" the comments of other Twitterers they've selected, and are themselves "followed.")
"You earn reputation, and then you give reputation. If lots of people follow you, and then you follow someone--then even though this [new person] does not have lots of followers," his tweet is deemed valuable because his followers are themselves followed widely, Singhal says. It is "definitely, definitely" more than a popularity contest, he adds.
Makes sense, as long as they can filter out the automated junk.