Tienes un ejemplo muy sencillo en la página de Facebook para desarrolladores. https://developers.facebook.com/docs/facebook-login/web
Espero que te ayude...
<!DOCTYPE html>
<html>
<head>
<title>Facebook Login JavaScript Example</title>
<meta charset="UTF-8">
</head>
<body>
<script>
function statusChangeCallback(response) {
console.log('statusChangeCallback');
console.log(response);
if (response.status === 'connected') {
testAPI();
} else if (response.status === 'not_authorized') {
document.getElementById('status').innerHTML = 'Please log ' +
'into this app.';
} else {
document.getElementById('status').innerHTML = 'Please log ' +
'into Facebook.';
}
}
function checkLoginState() {
FB.getLoginStatus(function(response) {
statusChangeCallback(response);
});
}
window.fbAsyncInit = function() {
FB.init({
appId : '{your-app-id}',
cookie : true, // enable cookies to allow the server to access
// the session
xfbml : true, // parse social plugins on this page
version : 'v2.5' // use graph api version 2.5
});
FB.getLoginStatus(function(response) {
statusChangeCallback(response);
});
};
(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/en_US/sdk.js";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));
function testAPI() {
console.log('Welcome! Fetching your information.... ');
FB.api('/me', function(response) {
console.log('Successful login for: ' + response.name);
document.getElementById('status').innerHTML =
'Thanks for logging in, ' + response.name + '!';
});
}
</script>
<fb:login-button scope="public_profile,email" onlogin="checkLoginState();">
</fb:login-button>
<div id="status">
</div>
</body>
</html>