Facebook and other social sites are monitoring all chats on their sites in search of criminal activity. If they deem your talk or correspondence suspicious, they notify the police.
Your conversations are monitored for simple “suspicious behavior,” behavior like vulgar language, personal information exchanges, age gaps, or any other of a myriad of reasons, that Facebook security employees deem as inappropriate or possibly criminal. Of course, this is all done without any cause or warrant, or even any initial involvement by the cretins who act as the state’s agents of force.
Facebook as well as others, work very closely with police, and act as a first wave of surveillance of their own customers. This is a privacy invasion, but then those who are willing to put their personal information out for any to see should not be surprised by this monitoring.
This is simply another cooperative venture between private companies and the state apparatus done at the expense of the users. This takes the “see something, say something” to another level. Nothing is private, and nothing is sacred, so before any use these social networks, be aware that you are being watched!