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.

It's auditable, open source, and immutable. It's neutral. Neither good nor evil. It's your responsibility or the community's responsibility to audit the code and assess the risks.

That's great at and all, but I honestly don't understand how that would help in this situation. Unless you write something absurd like "Mark Zuckerberg's stock will be transferred to Palmer Luckey if this promise is broken", which just about no one in Mark's position would agree to, how are you going to actually prevent something like this from happening?

edit: and as ComputerGuru stated - such a clause can just as easily be put into a traditional contract

So is a contract written in legalese English.

Will a blockchain contract block FB from committing the code in the Occulus software that requires to login in Facebook?

I guess it won't, so...

And this is why blockchains will never replace courts.