Several questions about continue with Facebook button

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Facebook-API

I am thinking about adding the Continue with Facebook button to my site but I have a few questions I need answering first before I screw up my website.

Members who login to my site basically only need a username and password also an email address.
My question are...

(1) When someone logs in from facebook do I need to add their details to my db as a newly registered member?

(2) My site only allows characters a-z A-Z 0-9 _ - in ther username, what is a facebook member uses different characters?

(3) My site does not allow spaces in the username, what if a connecting facebook member uses spaces?

(4) What is a Facebook member logs in to my site and has a name already used by one of my registered members?

(5) Usernames on my site can only be a max of 25 characters, what is a connecting facebook member has more than 25 characters in their username?

I do have other questions but can't remember than all right now. Any help would be great. Please don't send me links to read this or read that because usually none of it makes any sense to me and just wastes my day. thanks again

What I have tried:

I have not tried anything yet, I am looking for advice before I mess anything up