If you plan on buying the Galaxy S8 I recommend turning this option off entirely, at least if you want to keep prying Eyes from opening your smartphone. Just think – all someone needs to do is head to Facebook and grab a picture of you to unlock the device. That's terrifying.
You'll be much better off using the fingerprint reader or, better yet, the iris scanner. Princeton Identity, which provides the Galaxy S8's iris scanner technology, told Eyes On Events is an email that it's 100,000 times more resistant to false positives than other forms of biometric security, like facial recognition. It's able to achieve this by scanning 200 points of an iris during setup.
Keep this in mind with the Galaxy S8. I know I won't be using the facial recognition option.