The problem with block chain is it requires the whole stack be on chain (at least if it is to be referenced in a contract). If that's the case, then you can def

The problem with block chain is it requires the whole stack be on chain (at least if it is to be referenced in a contract). If that's the case, then you can definitely do this. E.g. a smart contract that represents the login functionality and you can hard-code it has to be some other immutable contract, and can't be changed to FB login. On second thought, the whole issue is weird because blockchain doesn't have a login concept.