I've heard something like that too. I know the police will try to befriend you on Facebook when they suspect you from any crime like drugs or something, and then they get lots of valuable information. That opens up a whole can of worms with privacy laws and such. Can't remember in which cases they were allowed to do this and where they can use the information from Facebook.
Alternatively that opens up a whole new range of trolling the police xD Post pictures of yourself with (fake) machine guns, explosives, drugs, etc. and then just wait for them to arrive xD