Authentication refers to verifying the identity of a user through either a trusted third-party service, or credentials provided to the user (such as a username and password). Registration is the process by which a user receives an account on your site (from the expert point of view, registration is the creation of a record for the user in the database).
When users visit your website for the first time, they should clearly realize that they are signing in (being registered). Using a third-party authentication system, they could be registered without their consent, if they are successfully authenticated through a third-party service. In general, this is not considered a good practice, and you need to make it clear to users that they register on your site or login (regardless of whether they pass third-party authentication) and provide a clear mechanism for canceling their membership.
One of the examples of user experience that you should consider is the confusion of third-party services. A user who registered in January using Facebook, and then returned in July and received an offer to log in via Google, Facebook, Twitter or LinkedIn, could forget long ago with which service he registered first. This is one of the pitfalls of third-party authentication, and here you can do very little. This is another good reason for asking users for their email address: so you give them the ability to find an account at an email address and send an email to him, specifying which service was used for authentication.
If you feel that you have a clear idea of the social networks that your users use, you can solve this problem through the main authentication service. For example, if you are sure that most of your users have a Facebook account, you can make a big button that says “Sign in with Facebook”. Then, using small buttons or even just text links, write “Or enter with Google, Twitter or LinkedIn”. This approach can significantly reduce the number of cases associated with confusion of third-party services.
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