What you're missing is that the sort of harassment in question isn't nasty names and unwanted messages on Facebook, it's using every piece of available informat

What you're missing is that the sort of harassment in question isn't nasty names and unwanted messages on Facebook, it's using every piece of available information to stalk and threaten the physical safety of the people in question. To the point of finding out where people live and following everyone they know personally who has a social media presence and posting things like, "Jane is at the bar downtown tonight, I hope someone kills that slut." Celebrities deal with this sort of thing by hiring bodyguards and other personal security teams. An average person cannot have that kind of protection.