Shoot five or more frames in quick succession, and Photos will analyze their content & try to make a stabilized, looping animation. You can also select any range of frames in the Android or iOS app, then tap the “+” icon and choose “Animation.”
To share it on Facebook, right-click the image itself, copy the URL, and paste it into Facebook. (I’ll keep pestering my friend there to make things a bit simpler still. :-))
Oh, and if you want to see all your animations & other creations (collages, etc.), visit the search page and tap “Creations” down at the bottom.