This wordpress plugin would need to work with the facebook’s graph API and requires user authentication. The admin could add some short code to a page and it would only show the content is the user clicked the lick button. The user would need to Like the page with the button shown before they could get access to the page. Once they liked it the content would become visible.
The plugin would have a graphic image that would be shown that would suggest that you need to like the page to get access. I will provide you with the image to uses. The admin can pick from 5 diff images to use.