We pumped into this issue before, here's the fix we arrived at after a while. It may or may not be suitable for your specific context, so think about it carefully before implementing.
There are two ways:
1. Allow the user to add email and password in the settings page, to be able to login using both social and email in the future.
2. Set the email associated with their social media as their account email (you have to notify the user about this in the settings page to avoid confusion) then add a "set password" button in the settings. Also, users must be allowed to edit the email if they want (after authentication of course). Once the user sets a password, they then can login with the email and password, as well as the social they signed up with.
To keep consistency across the users (those who signed up with email and those who used a social account) you should allow the user who signed up using email to connect social accounts from within the settings page. As they both should have the same freedom of logging in through social or email.
The two options must be thoroughly reviewed from a security and privacy point of view, as the limitations they pose can differ from one app to another and it might affect whether or not you can go with these fixes.