How to know if user's facebook's friends are registred on your site? - facebook

Question

I want to create a 'Find friends from facebook' button on my site. Many websites have it, and it basically gets your friends from facebook and identifies those of them who are already registred on that website, to allow you add them as friends or whatever.
I have clicked that button on a very famous website and the website show me a list with my friends who are registred on that website, and almost 1/3 of my friends where there.
I have been reading the facebooks API, have created an app, and finally managed to create a 'Find facebook friends' button. But when I click on that button, my website receives from facebook only an array with the name and fb ID of every friend I have.
How is that usefull for me? How can I know who of them are registred on my website to show it to the user that clicked the button? I thought it should give me for example the email of each friend, so I can compare it to my DB and get the registred users, but, now that I think of it again, it's true that it is impossible due to several fb privacy violations.
How do those websites know who of your friends are registred by clicking the button?
Thank you very much!

Answer 1

You need to associate a user's Facebook ID with their account on your site (in the user DB table for example).
Then you have the list of all Facebook IDs of your friends. You can run a query in your DB to see which of your registered users' Facebook IDs match the ones from your query.
Basically the intersection of 2 list of FB IDs.
The ones in both lists are your friends registered on the site.

Answer 2

If you're happy to add an extra permission to your FB authentication (publish_actions) then you you don't need to keep your own list of users in a DB to get this info - you can query the scores API for the app. Just make a Graph call to /APP_ID/scores and it will return a list of friends of the current user who are using this app. Your application doesn't even need to publish any scores info - in this scenario, the API will just return scores of 0.
The addition of publish_actions does mean that there will be an extra permission dialog appearing when users connect to your app/site if you are not already requesting this permission.
Only friends who have given your app/site the publish_actions permission will be shown in the list - so if you have an existing site and have not previously requested this permission then friends who are old users will not be shown. Totally understand this might not be suitable for you and perhaps you should implement Fabien Warniez's solution instead.
Javascript example:
FB.login(function(response) {
FB.api('/145634995501895/scores', function(scoresResponse) {
$.each(scoresResponse.data, function(key, value) {
console.log(value.user.id + ':' + value.user.name);
});
});
}, {scope: 'publish_actions'});
Replace 145634995501895 in the above code with your application ID.