Now, I found out the reason why it's doesn't appear. It seems to, user hasn't been logged in (it's appears after I signed-in). Hence, I have to let user sign-in first after the install for one-time, not multiple times. Perhaps, if there is other way around, such those buttons appear regardless signed-in, then it will ask for it after the click (it might discourage user for further social engagements...)
My question is, what is the best practice to sign-one for good? In other words. How to make one time Facebook sign-in?
Hopefully, it will address the issue. Thanks,