John Battelle asked Twitter founder, Evan Williams: “How do you pick ‘Who to Follow’?,”. During the course of his answer, Mr. Williams acknowledged that Twitter has a team of engineers and a system that creates a secret reputation score. They use that to suggest who to follow.
“We might make it public,” he said regarding the score, “But it has to evolve more.”
"Twitter has a (Secret) Reputation Score for every User", Alexia Tsotsis, TechCrunch, November 17, 2010.