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The basic concept is some sort of estimation based on who follows who. E.g. based on our current follower base, if you'd follow me that means I do something very well; if I follow you, that means nothing about me, and has smaller impact on you, too.

Hmm, interesting. Reminds me of the Google link authority algorithm, back in the day...

@scipio claims it was partially inspired by that. And I mean, it worked like a charm for Google!