To allow your users to login with their Facebook accounts, you must first setup an Application for your site:
- Visit developers.facebook.com/apps and select "Add a New App" from the "Apps" menu at the top.
- When the "Add a New App" window pops up, click the "advanced setup" link at the bottom.
- Type in a name (i.e. the name of your site), select a category, and click "Create App."
- Go to the "Settings" page and click "Add Platform," then "Website," then fill in your "Site URL." Note: http://example.com/ and http://www.example.com/ are not the same.
- Also on the "Settings" page, enter a Contact EMail, note the App ID and App Secret (you'll need them in a minute), and save changes.
- Go to the "Status & Review" page and make the app live (flip the switch at the top).
Then you can install the plugin:
- Download the latest version from here, unzip it, and upload the extracted files to your plugins directory.
- Login to your WordPress admin panel and activate the plugin.
- Navigate to Settings -> WP-FB AutoConn.
- Enter your App ID and Secret (obtained above), and click "Save."
- If you're using the default BuddyPress theme, a Facebook button will now automatically appear in its built-in login panel. Otherwise, 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.