While only iPhone 6s and iPhone 6s Plus can take a Live Photo, anyone running iOS 9 can post a Live Photo they've been sent. Viewing the Live Photos works the same in Facebook as it does the built-in Photos app: 3D Touch on the iPhones 6s and long press on any other iOS 9 device.
Unsure whether or not Live Photo support has been enabled on your Facebook account? Try posting a Live Photo but switch the visibility to "only me". That way you can test privately instead of in front of all your friends or the public.
If you do have Live Photos enabled let us know in the comments and tell us how it works for you!