As many people suggested here, uninstalling the updates bring the widget back...but the original app that preinstalled in the phone is very old.
As I have enabled auto-updates via Wifi, fb updates itself automatically when it is connected to my home wifi. The result - the widget disappeared again.
I use Galaxy Note 5. I've planned to write to facebook. I will post their suggestion later. In between, If any one has figured out, please let me know. Thanks guys.