1. Using OAuth to sign-up often means disclosing private data you can (and would normally prefer to) keep secret if you go the bare e-mail sign-up way. E.g. contacts list, exact date of birth, etc. - This is why I (as a user) stopped using OAuth for new accounts.
Kind of the same used to apply to e.g. Android apps. I mean the "give an app all the permissions it wants or gtfo" anti-pattern which ought be abolished. The user should be allowed to continue after denying/revoking access to any (but absolutely essential for the very function) data silently or manually specifying whatever values they want.
2. It isn't always easy to decouple an OAuth-based account from the social network account, especially in case you loose access to the latter. - This is why I (as a user) migrated all OAuth-based accounts I had to the good old e-mail way.