Things stop being opt-in when too many people start using them. There's a point where private platforms become so big they begin to function as public platforms

Things stop being opt-in when too many people start using them. There's a point where private platforms become so big they begin to function as public platforms. At that point, not opting in becomes tantamount to becoming an off-grid prepper, or religious hermit. Participating in society is technically opt-in as well, but because it is tied into all parts of life, not opting in requires you to opt out.