1- For excerpt (og:description) I use the below

if( !empty($Item) ){
								   $excerp = $Item->excerpt;
								// "Quotes" creates problems, replace em.
								  $quotz = array('"', '\'', '\"');
								  $none = array('"', '\'', '\"');
								  $excerpt = str_replace($quotz, "", $excerp);
								add_headline('<meta property="og:description" content="'.$excerpt.'">');
}

2- For og:image I use the attached image.

				if( !empty($Item) )
				{    // Make sure the Item is loaded
					if( $FileList = $Item->get_attachment_FileList(20) )
					{    // Get 20 attached files (there might be non-images so we try them all)
						while( $File = & $FileList->get_next() )
						{    // Loop through attached files
							if( $File->exists() && $File->is_image() )
							{    // Got an image
								//$url = $File->get_url();
								$url = $File->get_thumb_url( 'fit-852x480' );
								break;
							}
						}
					}
				}
				
				if( !empty($url) )
				{
					add_headline('<meta property="og:image" content="'.$url.'">');
				}

Also I'd like to point out that your og:url should not be prettyurl. Otherwise, you will lose your "likes" when the url changes -when you change your title or your permalink structure-. There's no way to redirect / transfer them. So I use the shortlink, such as:

		$id = $Item->ID;
		add_headline('<meta property="og:url" content="' . $baseurl .'index.php?p='.$id.'&blog='.$blog.'&redir=no">');

Plesae note that
1- These codes go to _skin.class.php ( I placed them inside my display_init() function )
2- You need to call the globals

global $blog, $disp, $Item, $baseurl, $skins_url;

3- You might wanna wrap your code with a conditional to display og tags only on single display to avoid confusion with a

if($disp == 'single')

4- This works with 4.x. With 5.x the og:image thing doesnt work since get_attachment_FileList() seems to have moved to another class

Speaking of which; does anyone know the best way to get the first attached image url of the post ?

I used to get it with the above code in 4.x

  // Make sure the Item is loaded
	                    if( $FileList = $Item->get_attachment_FileList(20) )
	                    {    // Get 20 attached files (there might be non-images so we try them all)
	                        while( $File = & $FileList->get_next() )
	                        {    // Loop through attached files
	                            if( $File->exists() && $File->is_image() )
	                            {    // Got an image
	                                //$url = $File->get_url();
	                                $url = $File->get_thumb_url( 'fit-852x480' );

In 5.x all I could do was getting the Images and preg_matching.

$post_images = $Item->get_images( $params,'raw');
preg_match('/(src)=("[^"]*")/',$post_images, $img_parts);
$postimgurl = str_replace(array('src="','"'),array('',''),$img_parts[0]);

This part seemed interesting and usable in _item.class.php but could not get it to work:

// Get list of attached files
		$LinkOnwer = new LinkItem( $this );
		if( ! $FileList = $LinkOnwer->get_attachment_FileList( $params['limit'], $params['restrict_to_image_position'] ) )
		{
			return '';
		}

		$galleries = array();

		/**
		 * @var File
		 */
		$File = NULL;

		while( $File = & $FileList->get_next() )
		{
			$params['File'] = $File;
if( ! $File->is_image() )
			{

// GET THE IMAGE URL HERE $File->get_attachment_url or something ?

For 5.x, here is what I used for og:tags in _skin.class.php

	function display_init()
	{
		global $blog, $disp, $Item, $baseurl, $skins_url, $Chapter, $ReqURI;	
		// call parent:
		parent::display_init();
			$og_uri = $_SERVER['HTTP_HOST'].$ReqURI;
			if( isset($Item) && ! empty($Item) ) {
				$og_uri = $baseurl .'index.php?p='.$id.'&blog='.$blog.'&redir=no';
				$og_desc = $Item->excerpt;
				$og_title = $Item->title;	
					$post_images = $Item->get_images( array(
						'before'              => '',
						'before_image'        => '',
						'before_image_legend' => '',
						'after_image_legend'  => '',
						'after_image'         => '',
						'after'               => '',
						'limit'               => 1,
						'image_link_to'       => 'original',
						'before_gallery'      => '',
						'after_gallery'       => '',
						// Optionally restrict to files/images linked to specific position: 'teaser'|'aftermore'
						'restrict_to_image_position' => 'teaser',
					),'raw' );
	
				if( ! empty ($post_images) ) {

					preg_match('/(src)=("[^"]*")/',$post_images, $img_parts);
					$postpic = str_replace(array('src="','"'),array('',''),$img_parts[0]);
				}
			}
			else if ( isset($Chapter) && ! empty($Chapter) ) {
				//if we are on category 
				$og_desc = $Chapter->description;
				$og_title = $Chapter->name;
			}
			else {
				// Just in case, for everything else.	
				$og_desc = $Blog->description;
				$og_title = $Blog->name;
			}
		$id = $Item->ID;
		add_headline('<meta property="og:type" content="article">');
		add_headline('<meta property="og:description" content="'.$og_desc.'">');
		add_headline('<meta property="og:url" content="'. $og_uri . '">');
		add_headline('<meta property="og:title" content="'.$og_title.'">');
		if ( isset($postpic) && ! empty($postpic) ) {
			add_headline('<meta property="og:image" content="'.$postpic.'">');
		}