How it works
Open the Facebook app. If you’ve taken more than five photos or videos within the past 24 hours, Facebook will show you a preview of a slideshow it has generated at the top of your News Feed.
Alternatively, if you’ve seen a friend share a slideshow to your News Feed, find the post and tap ‘Try It’. Tap on the preview to open a slideshow within Facebook and choose from the menu bar at the bottom of your screen. It will give you options: Library, Themes, Edit and More.