Let's say the user signs-up via Facebook. However, some time passed and the user forgot that they signed-in via Facebook and tried to log-in using their email.
Usually, for other apps, the login just won't validate since they never signed-up that way before (there was no password set). However, is this good UX? Is there another way to handle this (e.g. let them know that they already signed-in using Facebook?)?
If the person is trying to login from a different device you won't be able to tell him he already signed in via social-login.
What you can do though, is remind him that he might have used social-login before in the error message.
Something like "The e-mail or password you entered is not valid. Maybe you signed in by using social-login before?"
I think if your system supports both social media and email logins then you should allow both to be linked to the same account.
So the next time a user that has signed up through social login tries logging in through email, you can redirect him to sign-up screen saying that "there is no account with the given email id. Please try logging in via social login in case you created an account with that" (Do not mention if that user id already exists through social login since it is considered as a privacy/security breach -- some crawler can know which user's have signed up for your service and attack them via phishing).
This way, the user can still signup via email with a password and you can connect this to the social login usage, thereby allowing him to login via both options next time. If user logs in via email first and then tries to login via social login then its way simpler since you just need to allow him to do that.
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.
