Update: after Facebook shares manged to turn modestly green after the close, the have since slumped again, down over 3% after Zuckerberg said that in order to shift user activity to video and stories, it will have to take activity away from other kinds of social interactions which monetize a lot better. Why stories?
Because Zuckerberg wants Stories to be the thing that brings people back to the Facebook core app even though he admit the launch hadn't gone quite as well as expected, and as a result the company was prioritizing that kind of activity over the traditional Facebook feed of videos, posts and news. This is a significant change as "we don't make as much money from them yet as we do from feed ads."
And the red flag: he said 2019 will be another "investment year" as they figure out how to make the stories transition.