As people show signs of gravitating away from Facebook, Mark Zuckerberg says his personal challenge for 2018 is to fix what ails the giant social network. The focus is no longer on how much time its 2 billion-plus users spend on Facebook, but whether that time is “well spent”. So far it seems Mr. Zuckerberg’s measures are working as time spent on Facebook has been reduced by roughly 50 million hours every day.