What oauth grant type should I use after social sign in

I have an application which has its own oauth microservice to issue access tokens. For users registering using email and password I am using grant_type = 'password'. What should I do when a users signs in using social media?

I thought of generating a random password for getting access tokens, but the oauth server accepts only plain text passwords and I am storing password as hashes in my db. Should I be using a different grant type to get tokens?