Correct way in 2018 to open Facebook app via website link?

I figured it out, hope it helps someone else.

If the device is Android, use:

fb://facewebmodal/

And if it's iOS, use:

fb://page/PAGEID

If you do it automatically you need to detect the device first and then redirect to the appropriate link.

Now it's changed to:

For iOS: fb://profile/PAGEID

For Android: fb://page/PAGEID

PAGEID is the numeric pageid that the page owner can see in the about box in facebook