Add the below og details to your product details page:
<meta property="og:description" content="Your content to share on facebook" />
<meta property="og:image" content="http://www.example.com/media...productimage.jpg" />
Replace description and image url with your product description and image.
For more guidance you can refer the below link:
https://developers.facebook.com/docs/sharing/best-practices#tags
Add the below code snippet to your head.phtml
<?php
$_prodID = Mage::registry('current_product')->getId();
$_product = Mage::getModel('catalog/product')->load($_prodID );
?>
<meta property="og:title" content="<?php echo $_product->getog_title() ?>" />
<meta property="og:type" content="product" />
<meta property="og:image" content="<?php echo $_product->getog_image() ?>" />
<meta property="og:url" content="<?php echo Mage::registry('product')->getProductUrl(); ?>" />
<meta property="og:description" content="<?php echo $_product->getog_description() ?>" />
<meta property="og:site_name" content="<?php echo Mage::app()->getStore()->getName(); ?>" />