Spy service
Facebook was the closest thing to such a thing other than the FBI tapping your phone. Except nobody knew it. When I was a teenager I thought it was silly and fun and enjoyed starting trivial pages and petitions. I shared a lot of personal information from 2008-2014. I had not a clue how serious it was to do such a thing of what the websites purpose was.
Now I get it. I know there are people that give off tips of doing something violent on social media and then the act can be intervened because they're able to tell who they are. But the majority of people weren't doing anything illegal or never intended to.