backends.py

from django.conf import settings
from django.contrib.auth import models as auth_models

import cgi
import urllib
import simplejson 

from <app_name> import models

class FacebookBackend:

    def authenticate(self, token=None):

        facebook_session = models.FacebookSession.objects.get(
            access_token=token,
        )

        profile = facebook_session.query('me')

        try:
            user = auth_models.User.objects.get(username=profile['id'])
        except auth_models.User.DoesNotExist, e:
            user = auth_models.User(username=profile['id'])

        user.set_unusable_password()
        user.email = profile['email']
        user.first_name = profile['first_name']
        user.last_name = profile['last_name']
        user.save()

        try:
            models.FacebookSession.objects.get(uid=profile['id']).delete()
        except models.FacebookSession.DoesNotExist, e:
            pass

        facebook_session.uid = profile['id']
        facebook_session.user = user
        facebook_session.save()

        return user

    def get_user(self, user_id):

        try:
            return auth_models.User.objects.get(pk=user_id)
        except auth_models.User.DoesNotExist:
            return None

login.html

 {% if error %}
      {% if error == 'AUTH_FAILED' %}
          <p>Authentication failed</p>
      {% else %}{% if error == 'AUTH_DISABLED' %}
          <p>Your account is disabled</p>
      {% else %}{% if error == 'AUTH_DENIED' %}
          <p>You did not allow access</p>
       {% endif %}{% endif %}{% endif %}
  {% else %}
    <a href="https://graph.facebook.com/oauth/authorize?client_id={{ settings.FACEBOOK_APP_ID     }}&redirect_uri={{ settings.FACEBOOK_REDIRECT_URI }}&scope=publish_stream,email&display=popup">
      <img src="http://developers.facebook.com/images/devsite/login-button.png"/>
    </a>
  {% endif %}

models.py

from django.db import models
from django.contrib.auth.models import User

class FacebookSessionError(Exception):
    def __init__(self, error_type, message):
        self.message = message
        self.type = error_type
    def get_message(self):
        return self.message
    def get_type(self):
        return self.type
    def __unicode__(self):
        return u'%s: "%s"' % (self.type, self.message)

class FacebookSession(models.Model):

    access_token = models.CharField(max_length=103, unique=True)
    expires = models.IntegerField(null=True)

    user = models.ForeignKey(User, null=True)
    uid = models.BigIntegerField(unique=True, null=True)

    class Meta:
        unique_together = (('user', 'uid'), ('access_token', 'expires'))

    def query(self, object_id, connection_type=None, metadata=False):
        import urllib
        import simplejson

        url = 'https://graph.facebook.com/%s' % (object_id)
        if connection_type:
            url += '/%s' % (connection_type)

        params = {'access_token': self.access_token}
        if metadata:
            params['metadata'] = 1

        url += '?' + urllib.urlencode(params)
        response = simplejson.load(urllib.urlopen(url))
        if 'error' in response:
            error = response['error']
            raise FacebookSessionError(error['type'], error['message'])
        return response

settings.py

FACEBOOK_APP_ID = ''
FACEBOOK_API_KEY = ''
FACEBOOK_API_SECRET = ''
FACEBOOK_REDIRECT_URI = 'http://example.com/login/'

AUTHENTICATION_BACKENDS = (
    '<app_name>.backends.FacebookBackend',
)

views.py

from django.contrib import auth
from django.http import HttpResponseRedirect
from django.shortcuts import render_to_response
from django.template import RequestContext

import cgi
import simplejson
import urllib

from <app_name> import settings

def login(request):
    error = None

    if request.user.is_authenticated():
        return HttpResponseRedirect('/yay/')

    if request.GET:
        if 'code' in request.GET:
            args = {
                'client_id': settings.FACEBOOK_APP_ID,
                'redirect_uri': settings.FACEBOOK_REDIRECT_URI,
                'client_secret': settings.FACEBOOK_API_SECRET,
                'code': request.GET['code'],
            }

            url = 'https://graph.facebook.com/oauth/access_token?' + \
                    urllib.urlencode(args)
            response = cgi.parse_qs(urllib.urlopen(url).read())
            access_token = response['access_token'][0]
            expires = response['expires'][0]

            facebook_session = models.FacebookSession.objects.get_or_create(
                access_token=access_token,
            )[0]

            facebook_session.expires = expires
            facebook_session.save()

            user = auth.authenticate(token=access_token)
            if user:
                if user.is_active:
                    auth.login(request, user)
                    return HttpResponseRedirect('/yay/')
                else:
                    error = 'AUTH_DISABLED'
            else:
                error = 'AUTH_FAILED'
        elif 'error_reason' in request.GET:
            error = 'AUTH_DENIED'

    template_context = {'settings': settings, 'error': error}
    return render_to_response('login.html', template_context, context_instance=RequestContext(request))