I suspect that Facebook still wants to know that some letters have a different case than other letters. If They just converted all letters to uppercase then that information would be lost.
> ie. compare both hash(enteredPassword) and hash(invertCase(enteredPassword)) to the hash in the database and see if either match
Here's an example in bash:
$ echo PasSwoRd | tr 'A-Za-z' 'a-zA-Z'
pASsWOrD
$ echo PasSwoRd | tr 'A-Za-z' 'a-zA-Z' | sha256sum > db
$ echo PasSwoRd | sha256sum > sum1
$ echo PasSwoRd | tr 'A-Za-z' 'a-zA-Z' | sha256sum > sum2
$ diff -q db sum1
Files db and sum1 differ # i.e. LOGIN FAILS
$ diff -q db sum2
$ # i.e. LOGIN SUCCEEDS