Nicholas D. Kristof enjoys a prominent perch in American life; he’s a columnist for the New York Times, where he can reach millions of people every week. But he’s also on Facebook, where he has almost 20,000 fans. Why Facebook? The principle is simple: for maximum influence, go where the eyeballs are. According to Alexa.com, Facebook is the #5 most visited website in the U.S.; NYTimes.com is #21.