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To allow your users to login with their Facebook accounts, you must first setup an Application for your site:

1. Visit developers.facebook.com/apps and click the "Create New App" button.
2. Type in a name (i.e. the name of your blog). This is what Facebook will show on the login popup.
3. Facebook may now require you to verify your account before continuing (see here for more information).
4. Once your app has been created, fill in your "Site URL" under "Select how your app integrates with Facebook -> Website". Note: http://example.com/ and http://www.example.com/ are not the same.
5. Click "Save Changes," and note the App ID and Secret (you'll need them in a minute).

Then you can install the plugin:

1. Download the latest version from here, unzip it, and upload the extracted files to your plugins directory.
2. Login to your WordPress admin panel and activate the plugin.
3. Navigate to Settings -> WP-FB AutoConn.
4. Enter your App ID and Secret (obtained above), and click "Save."
5. If you're using BuddyPress, a Facebook button will automatically be added to its built-in login panel. If not, navigate to Appearance -> Widgets and add the WP-FB AutoConnect widget to your sidebar.

That's it - users should now be able to use the widget to login to your blog with their Facebook accounts.

For more information on exactly how this plugin's login process works and how it can be customized, see the homepage.