If you only use social logins, no guest license is consumed; it is treated as an application authentication within ClearPass. If you also have other means of guest access, those accounts will still need to abide by the guest licensing structure.
EDIT:
To elaborate. ClearPass Guest keeps count of each unique device that authenticates against the system using an account that exists in the Guest User DB. So social logins do not count as the account does not exist in the Guest DB.