We're venturing a bit off topic here, but yes I can understand a user with many logins on many sites may not remember which they used on yours. You can't really remind them on failed login without subjecting yourself to leaking that a user is a member of your service (or full blown user enumeration attacks).

In these cases where social logins are the primary approach most users use, I suggest making the email login look like a social login button, but make it clear it's email (i.e. more than just an icon) and trust only the few I-never-use-social-login users will leverage it while not confusing the others.