Restart your phone or other mobile device; -If you are using a phone, please uninstall and reinstall the application; -Log in to Facebook and try again. Also, please make sure that you are connected to a secure Wi-Fi network and the connection is stable.
Try to force close the app and then restart the phone. If that doesn't work, try rebooting your phone. If none of those work, try uninstalling Facebook and reinstalling it.