Glossary
Privacy policy
A privacy policy is the document where a platform states in writing what data it collects, why, who it passes that data to, how long it keeps it, and how you can ask to see or erase it. It is the platform's own account of itself rather than a neutral description, but it is the only version of these answers you can hold it to.
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Where you see it
A footer link on every page, and again beside the tick box at sign-up, where agreeing to it is usually bundled with agreeing to the terms of service.
Why it matters
Two sections carry most of the weight: who the data is shared with - payment processors, advertising partners, analytics providers - and how long records are kept after an account closes, which is normally longer than the account itself. Reading those two before you register tells you more than the rest of the document. Policies also change, and the version you agreed to is not the version that applies later.