Facebook Login and Facebook Registration Plugins

Facebook documentation and support is so bad that I come to StackOverflow to get answers :)

My question is about usability. When a user uses Facebook Login, can I "register" them on my site? Or is that unethical even if I can get their data?

Should I make them use the Register With Facebook plugin in order to get their data and make an account for them on my sit开发者_StackOverflow社区e?

If so, then in the future when the users login with Facebook, how do I know if they have an account on my site or not?

And if I show 2 buttons: Register and Login - what if they always log in and never register?

Thanks, Alex

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1: Yes you can. You get a lot of information from facebook if you want...

2: Just log them in as soon as they visit your page. You should get the facebook uid as soon as they are logged in with facebook and boooom you know who's on your site.