They’re my goddamn bits, that’s what’s the matter.

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.

That's it none of their fucking business and that it's quite likely a violation of the GDPR.

Nevertheless there's absolutely nothing I can do about it.

Your question reminds me of the "If you have nothing to hide " fallacy