[AC Question]

How do I remove Facebook from Galaxy S 6 ?

[hallux]

If it was there when you first booted the phone, your only option is likely to just disable it. In the app drawer, long-press on the Facebook icon and drag it to App Info. This takes you to a place where you can disable the app (or uninstall it if it's possible). When disabling the app, you'll need to uninstall any updates prior to disabling the app.

[Rukbat]

And delete all data before removing it, or the data will be there as a "ghost" and you'll never reclaim that space.