Please use the following code as a text link or image link in your email template: 
http://www.facebook.com/share.php?u=YourURLGoesHere

To include an actual Facebook share button, you may use this tool to generate script to display the share button with your desired link: https://developers.facebook.com/docs/plugins/share-button

Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.