As the new school year gets fully under way and presidential hopefuls work the young voter circuit via social networking websites, one online group — Facebook’s “Stop Hillary Clinton (One Million Strong AGAINST Hillary)” — is seriously outpacing its pro-candidate rivals in attracting new members. With more than 418,000 members, Stop Hillary Clinton has now surpassed “Barack Obama (One Million Strong for Barack)” as the largest group for or against a presidential candidate on Facebook. (The pro-Obama group has 355,000 members.) The anti-Clinton group is growing at 10 percent per month, said Micah Sifry, co-director of Tech President, a blog...