The privacy question behind 'Sign in with Facebook' is not whether the button works. It works. The real question is what kind of identity bridge it creates between a merchant and a social graph. Meta's login docs show that the flow is designed to let apps authenticate users and request permissions. That can be useful for convenience, but it also means the shopping site is not getting a random, disposable identity. It is often getting a login path that sits next to a much larger profile system.
That is the danger for normal shoppers. A store account created through a social login can become easier to recognize across sessions, devices, and campaigns than a guest checkout ever would be. Once a merchant can reliably say, 'this is the same person who returned last week and also used the app,' it can start to connect browsing, purchases, support messages, and ad responses into one record. The risk is not only that the merchant sees more. It is that the merchant sees the same person more clearly over time.
Meta's permissions model is part of the story because access is not automatic magic; it is a set of requested scopes and disclosures. But that does not remove the privacy cost. It just relocates the decision into a consent screen that many users click through quickly. If a shopping app asks for a Facebook login to avoid building its own account system, the user can end up paying with a durable identity token instead of a password. The token may be technically secure while still being privacy-expensive.
The FTC's personal-information guidance says businesses should know what they collect, control access, and limit retention. That is especially important when social logins are used for commerce because the temptation is to turn identity into a growth channel. Merchants should not quietly reuse the login to expand ad targeting or build shadow profiles beyond what the customer expects. If the site only needs a receipt and order history, then it should not pretend the social graph is part of the checkout requirement.
NIST's federation guidance is useful again here: federation can improve convenience and security, but it can also concentrate trust and identity in one upstream account. That can be fine when the user wants a stable account. It is not fine when the merchant nudges the shopper into social login just to avoid offering a guest path. Social sign-in should be an option, not a default trap that makes opting out feel like a broken experience.
For shoppers, the practical defense is to use Facebook login only when you really want the account continuity and the merchant has a legitimate reason to keep it. Otherwise, guest checkout, email aliasing, or a dedicated shopping inbox may be better. If the merchant asks for extra profile permissions, stop and read what is being requested. A login that saves thirty seconds is not automatically worth the long-term link between your social identity and your shopping behavior.
cloak should surface this as a profile-linking risk. The button itself is not the issue. The issue is the merger of social identity, app identity, and commerce identity into one easy corridor. Anti-exploitation privacy means making that corridor visible so the user can choose whether convenience is worth the link. If a store wants the certainty of a social login, it should earn it with a clear explanation, not hide it behind a shiny blue button.
There is a compounding effect too. Once a merchant uses social login, it becomes easier to ask for the next little piece of identity: a phone number for alerts, a profile field for personalization, a birthday for offers, or app permissions for device access. Each ask may look small. Together they turn a simple checkout into a layered profile that is easier to target, resell, or retain than the shopper intended when they clicked the login button.