Facebook "Like" button lets users share a web page with their friends. When a user clicks the Like button on any site, a story appears in the user's friends' news feed with a link back to the web site hosting the clicked Like button.
Having seen and purchased your deals, customers will be happy to share them with their friends. So "Like" button helps people to spread the word about your site and cool deals it serves.
If you have already set up your Facebook Application, you would then need to enable "Facebook sharing" option under Facebook tab at The Settings page in Admin section. The "Like" button will appear automatically on every deal you publish.