Another month, another Twitter milestone for the Biebs. The singer hit the 25-million-follower mark on the social media site this month. But that's not all he accomplished online in July. Justin signed on to invest in iPhone and iPad recommendations app Stamped, which helps you keep track of and share favorite restaurants, books, music, movies and apps. Stamped also lets you see what A-list tastemakers recommend. Would you give this app your stamp of approval?