How is Facebook responsible for a misuse of information that its users voluntarily disseminated into the public domain?

A great many people freely share their political views on facebook, along with all sorts of other personal information. Why is it facebook's responsibility to guard information the individuals themselves don't guard. Facebook isn't a utility company divulging information given to them via a contract for services, people put their information on facebook voluntarily knowing the security risks posed by their doing so. I don't get it. Shouldn't the anger be directed at the firms harvesting and selling this information?