Facing is an optional rule presented in the 5e Dungeon Master's Guide (page 252), flanking is an optional rule on the preceding page. Both are means of generating advantage in combat, they just aren't standard rules for simplicity.
I think there are times when facing makes sense, such as determining whether your shield is effective when shot at, but in melee combat, unless they are also flanked, an enemy should be able to turn as their opponent circles them. IMO the backstab status should only be possible if your are able to run/sneak up behind someone from further away, not by circling an engaged opponent.
