Facebook is essentially using the same techniques to monitor private communications as the NSA supposedly does. This means Facebook has the power to report, for example, selected messages but not others. (I'm not saying they do, of course, just that they could be selective or discriminatory that way.)
The fact is that Facebook has taken upon itself a role similar to that of the police, but without any democratic oversight.
This is different from a bar owner overhearing a conversation about a crime and calling the police, because he wasn't specifically monitoring every single word said by every bar patron. But Facebook is casting a wide net by analyzing every conversation that happens.
Questions: should Facebook be permitted to do this? Should we ask for laws preventing companies from "eavesdropping" on their users' communications with the intent of detecting and reporting criminal behavior? Should this be the role of the democratically-elected government instead? Should sites be required to turn user communication over to the government for such analysis?
It's a fascinating area of law/politics with so much room for future development, and gets down to the heart of what values a society has.