Exactly, Google, FB, etc. They require you login to their website. They also now know you use web app X, the date/time, how often you use said website, etc.
For some applications, that privacy loss may not be a big deal. Except if you combine this information with the other 500 web apps the user also uses through 'Sign in with...' links, plus all the other information they gather, they suddenly get to know you really, really well.
For some applications, that privacy loss may not be a big deal. Except if you combine this information with the other 500 web apps the user also uses through 'Sign in with...' links, plus all the other information they gather, they suddenly get to know you really, really well.