Multi login in codeignter

Hello guys thanks for answering questions few days ago, i have system multi ( register, login and logout ) using facebook, twitter, (manual input type information users )

here is my table --> id_user, email, name, password  (only thi field is important)

i have success

1. Create register, login and logout manual in codiegnter by input some field
   
2. Create login facebook without insert data users to database
   

what i have not create

1. Login Twitter 
   

My question

1. how possible when users register by facebook then login in twitter or other
   
2. am i wrong to said that to my boss when users register by facebook they only can login by facebook too, when they are trying another way to login it cannot
   
3. when user register in facebook or twitter can i store their email and id_user (generate by auto increment)
   
4. let say when user login by facebook or twitter when input master products can i get their id_users
   

this system need the id from users so it give me blank    idea firt time

IF you guys have experience about this please share something idea to me as new bie especially Codeignter 3