And they can have easy ways of tracking popularity. You can use Google Analytics and similar products in compliant ways, and the tools for that have existed for ages (at least in the case of GA).
You seem to misunderstand that you're entering into a partnership with a third party. They become a data processor on your behalf. They process a whole lot more, and you have access to a lot of that data. Fortunately, the social media platform includes the privacy policy and consent process in their onboarding of users, so you don't need to worry about it for the purposes of social login.