Facebook COO Sheryl Sandberg said that 20.8 million people now use Facebook on mobile devices, and that mobile users are far more active than those who don’t use it on their mobile phone. In November, Facebook reported 15 milllion mobile users. The social-networking site has about 150 million users overall. Sandberg announced the figure at the Palm Keynote, where she represented Facebook as one of Palm’s partners for its new phone/OS.