While catching child predators and murderers is a good thing, I can't help but consider that if facebook operated in china, they'd be catching political dissidents, or if facebook operated in uganda, they'd be catching homosexual users.
Doesn't that make them liable for anything they miss?
Eg: they're scanning chats for discussions about criminal activities, and reporting what they find to the police. If some people plan a murder in Facebook chat, it goes unnoticed, and they commit the murder, the victim's family could potentially sue Facebook for providing a forum for the planning to take place. Facebook can no longer claim that they're not responsible for the actions of their users, because they're actively monitoring those actions and reporting to the police.
The victim's family could potentially sue Facebook for providing a forum for the planning to take place, but is any judge going to rule in favour of the family? I think you'd have to establish that if Facebook Chat hadn't existed, then the murder never would have been planned (as opposed to the criminals simply using some other channel of communication such as email or talking on the phone).
Just because Facebook is monitoring chat sessions, I don't think it's been established that that makes them liable for any activities which are planned over those chat sessions. They're augmenting the police, not replacing them.
I guess I was thinking of Common Carrier status [0]:
The FCC classified Internet Service Providers as common carriers, effective June 12, 2015, for the purpose of enforcing net neutrality. Before that time, the Good Samaritan provision of the Communications Decency Act established immunity from liability for third party content on grounds of libel or slander, and the DMCA established that ISPs that comply with the DMCA would not be liable for the copyright violations of third parties on their network.
My understanding (which may be flawed) is that "ISP" covers things like discussion forums, public chatrooms, and private(ish) chat like Skype and Facebook Messenger, and that the liability immunity goes beyond libel and slander. But, ISPs that filter third-party content are not protected because they're exerting control over the content, and implicitly approve of anything that is not filtered.