Earlier quoted context omitted.
> Signed agreements do not prevent your information from being used in other ways. Why is this, though? How is it different from any other legal agreements?
I think the argument is that saying "I promise not to murder someone" does not in any way prevent said party turning around and murdering someone - signing a contract does not prevent these companies from violating the terms of the contract.
Companies share sensitive information all the time, this is not a new thing, contracts are real things, breach of contracts is a big deal, consequences can be huge.
Both FB and most of the entities with whom they would have shared data are huge, and there'd be way to much risk in not being smart with the data.