Chinese face-swap app Zao rocketed to the top of app store charts over the weekend, but user delight at the prospect of becoming instant superstars quickly turned sour as privacy implications began to sink in. Launched recently, Zao is currently topping the free download chart on China's iOS store. Its popularity has also pushed another face-swap app, Yanji, to fifth place on the list. Behind Zao is a company fully owned by Chinese hookup and live-streaming service Momo Inc. Users of the app upload a photo of themselves to drop their likeness into popular scenes from hundreds of movies or TV shows. It's a chance to be the star and swap places with the likes of Marilyn Monroe, Leonardo DiCaprio or Sheldon Cooper from The Big Bang Theory in a matter of moments.