I have used my Facebook account and Gmail account to access dozens of sites.
How can I find out on which sites I've used these authentication methods?
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You are asking two similar questions. My answer will try and address both.
To see which sites have used Google Authentication:
To see which sites have used the Facebook Authentication:
More info: Facebook uses a variant of the OAuth2.0 protocol. Google+ Sign-In is built on the OAuth 2.0 and OpenID Connect protocols.