Is it OK to create a user account on website for user while he presses the 'Login with Facebook' button to view a website?

Yes, absolutely.  Most modules like omniauth-facebook (Rails) suggest this (find_or_create).

Yes! It is one of the main benefits of using a social login (Such as Facebook). You can create an avatar and profile for them based on the data on their Facebook profile. This makes it much easier for the user and increases their user experience. It creates a more personalized experience for them, while giving you great first hand insights about them!

Yes it is OK do that, just make sure that you wont ask for too many permissions unless they are extremely needed. You can ask for more permissions later in the flow reason being most people wont likely grant the access in the beginning itself. You can find more details and how to optimize permission requests here https://developers.facebook.com/docs/facebook-login/permissions/v2.1#optimizing e.g. this https://coursi.co/

Is it Ok? It is necessary ! Where are you going to store user-specific data if you do not make accounts for Facebook users? A Facebook user should be similar to a normal user in your database structure, with the only difference that you have his 'Facebook ID' stored with you, using which he can directly login.

In most of the cases where the user is making some kind of transaction or creating personal data, a user account is must. Like if you see Quora after you login with Facebook or Twitter it creates an additional account, since it captures the basic details from these social media site and you don't need to enter them again. I personally don't understand this logic, so whether you login with facebook or directly, a user account would be created anyways, hence why do services promote login with facebook or twitter. I could only see certain benefits like posting their buying information on social media website.