Facebook is one of the biggest websites in the world. Only Google is currently higher according to Alexa.
This means there are a lot of people on the website everyday, and not everyone is being who they make out to be. Fake profiles are skyrocketing, and there is all sorts of problems happening including grooming and much more. Facebook tries to keep a lot of this out of the media to avoid the bad press and lose users, but most people know that it is the case.