It doesn't remove even a single bit of entropy. For an offline attack, where you have to attack Facebook's bespoke "password onion"[1], you would still have to

It doesn't remove even a single bit of entropy. For an offline attack, where you have to attack Facebook's bespoke "password onion"[1], you would still have to compute both hashes. There is approximately zero impact on cracking strategy. There may be an argument to be made that Facebook would have otherwise used a larger work factor had they not had to do double the work for wrong passwords, but the common case is the password being correct, so I doubt this was a significant consideration in the parameter selection. Even for the online case, I really don't think it affects security in a nontrivial way. 1. https://bristolcrypto.blogspot.com/2015/01/password-hashing-according-to-facebook.html https://bristolcrypto.blogspot.com/2015/01/password-hashing-...