I suppose the general fear is that we are the product and we don’t know what we are being sold for or to whom. If they are so sophisticated they can infer behavioral patterns after you quit by using your still existing network of friends who didn’t quit then it’s a pretty useful bit of profiling technology with buyers. The only part that ever really mattered is how they monetize the product, us. It’s an unspoken agreement for using a service which has no transparent monetary cost.
So it’s fear of the unknown? I’m genuinely curious why people are so bothered by this. I don’t use Facebook, and use privacy extensions to prevent my agents from using Facebook. It’s like they’re a non existent entity in my world.
I simply can’t prioritize caring about something that doesn’t appear to impact my life in any way. But I’m admittedly not much of a principalled-living person. Too much internal hypocrisy for me to pretend to be principled.