But Facebook/Twitter/etc isn’t telling out boss what we say about them at home. Or our SSN to every car we pass. Or how much we hate it family-in-law.
Unless you give them realistic reasons, they will always counter with “That’s different” because it is.
Personally, I’ve always thought “They could sell your data to insurance companies who might jack up your prices or claim something is a pre-existing condition” to be the most p we persuasive to people like my parents.