$loginUri = "https://graph.facebook.com/oauth/authorize?client_id={$appId}&redirect_uri={$redirectUri}&scope={$scope}";
header('Location: ' . $loginUri);

$client = new Zend_Http_Client( "https://graph.facebook.com/oauth/access_token" );
$client->setParameterGet('client_id', $appId);
$client->setParameterGet('client_secret', $secret);
$client->setParameterGet('code', $code);
$client->setParameterGet('redirect_uri', $redirectUri);

$result = $client->request('GET');
$params = array();
parse_str($result->getBody(), $params);
$token = $params['access_token'];

$client = new Zend_Http_Client( "https://graph.facebook.com/me" );
$client->setParameterGet('client_id', $appId);
$client->setParameterGet('access_token', $token);
$result = $client->request('GET');
$user = json_decode($result->getBody());

file: Zend/Auth/Adapter/Facebook.php

class Zend_Auth_Adapter_Facebook implements Zend_Auth_Adapter_Interface
{
    /**
     * The Authentication URI, used to bounce the user to the facebook redirect uri.
     * 
     * @var string
     */
    const AUTH_URI = 'https://graph.facebook.com/oauth/authorize?client_id=%s&redirect_uri=%s';
    
    /**
     * The token URI, used to retrieve the OAuth Token.
     * 
     * @var string
     */
    const TOKEN_URI = 'https://graph.facebook.com/oauth/access_token';
    
    
    /**
     * The user URI, used to retrieve information about the user.
     * 
     * @var string
     */
    const USER_URI = 'https://graph.facebook.com/me';
    
    /**
     * The application ID
     *
     * @var string
     */
    private $_appId = null;

    /**
     * The application secret
     *
     * @var string
     */
    private $_secret = null;

    /**
     * The authentication scope (advanced options) requested
     *
     * @var string
     */
    private $_scope = null;

    /**
     * The redirect uri
     *
     * @var string
     */
    private $_redirectUri = null;

    /**
     * Constructor
     *
     * @param string $appId the application ID
     * @param string $secret the application secret
     * @param string $scope the application scope
     * @param string $redirectUri the URI to redirect the user to after successful authentication
     */
    public function __construct($appId, $secret, $redirectUri, $scope)
    {
        $this->_appId = $appId;
        $this->_secret = $secret;
        $this->_scope = $scope;
        $this->_redirectUri   = $redirectUri;
    }

    /**
     * Sets the value to be used as the application ID
     *
     * @param  string $appId The application ID
     * @return Zend_Auth_Adapter_Facebook Provides a fluent interface
     */
    public function setAppId($appId)
    {
        $this->_appId = $id;
        return $this;
    }

    /**
     * Sets the value to be used as the application secret
     *
     * @param  string $secret The application secret
     * @return Zend_Auth_Adapter_Facebook Provides a fluent interface
     */
    public function setSecret($secret)
    {
        $this->_secret = $secret;
        return $this;
    }

    /**
     * Sets the value to be used as the application scope (array())
     *
     * @param  string $scope The application scope
     * @return Zend_Auth_Adapter_Facebook Provides a fluent interface
     */
    public function setApplicationScope($scope)
    {
        $this->_scope = $scope;
        return $this;
    }

    /**
     * Sets the redirect uri after successful authentication
     *
     * @param  string $redirectUri The redirect URI
     * @return Zend_Auth_Adapter_Facebook Provides a fluent interface
     */
    public function setRedirectUri($redirectUri)
    {
        $this->_redirectUri = $redirectUri;
        return $this;
    }


    /**
     * Authenticates the user against facebook
     * Defined by Zend_Auth_Adapter_Interface.
     *
     * @throws Zend_Auth_Adapter_Exception If answering the authentication query is impossible
     * @return Zend_Auth_Result
     */
    public function authenticate()
    {
    	// Get the request object.
    	$frontController = Zend_Controller_Front::getInstance();
    	$request = $frontController->getRequest();
    	
    	// First check to see wether we're processing a redirect response.
    	$code = $request->getParam('code');
    	
    	if ( empty ($code ) )
    	{
	    	// Create the initial redirect
	    	$loginUri = sprintf(Zend_Auth_Adapter_Facebook::AUTH_URI , $this->_appId, $this->_redirectUri);
	 		
	    	if ( !empty($this->_scope) )
	    	{
	    		$loginUri .= "&scope=" . $this->_scope;
	    	}
	    	
	    	header('Location: ' . $loginUri );
    	}
    	else
    	{
    		// Looks like we have a code. Let's get ourselves an access token
	    	$client = new Zend_Http_Client( Zend_Auth_Adapter_Facebook::TOKEN_URI );
	    	$client->setParameterGet('client_id', $this->_appId);
	    	$client->setParameterGet('client_secret', $this->_secret);
	    	$client->setParameterGet('code', $code);
	    	$client->setParameterGet('redirect_uri', $this->_redirectUri);
	    	
	    	$result = $client->request('GET');
	    	$params = array();
	    	parse_str($result->getBody(), $params);
	    	
	    	// REtrieve the user info
	    	$client = new Zend_Http_Client(Zend_Auth_Adapter_Facebook::USER_URI );
	    	$client->setParameterGet('client_id', $this->_appId);
	    	$client->setParameterGet('access_token', $params['access_token']);
	    	$result = $client->request('GET');
	    	$user = json_decode($result->getBody());
	    	
            return new Zend_Auth_Result( Zend_Auth_Result::SUCCESS, $user->id, array('user'=>$user, 'token'=>$params['access_token']) );
    	}
    	
        return new Zend_Auth_Result( Zend_Auth_Result::FAILURE, null, 'Error while attempting to redirect.' );
    }
}

file: modules/default/controllers/FacebookauthController.php

/**
 * Sample Facebook Auth Controller.
 * 
 * @author Michael Krotscheck
 */
class FacebookauthController extends Zend_Controller_Action
{
	/**
	 * Application default action
	 */
	public function indexAction ()
	{
		$appId = 'YOUR_APP_ID';
		$secret = 'YOUR_APP_SECRET';
		$redirectUri = 'http://path-to-this-controller-and-action';
		$scope = 'YOUR_COMMA_SEPARATED_APPLICATION_SCOPE';
		
		// Create the authentication adapter.
		$adapter = new Zend_Auth_Adapter_Facebook( $appId, $secret, $redirectUri, $scope );
		
		// Get an authenticator instance
		$auth = Zend_Auth::getInstance();
		
		// This call will automatically redirect to facebook with the passed parameters.
		$result = $auth->authenticate($adapter);
		
		if ( $result->isValid() )
		{
			// Get the messages
			$messages = $result->getMessages();
			
			// Get the user object from the returned messages.
			$fbUser = $messages['user'];
			
			var_dump($fbUser);
		}
	}
}