Hi, i have two plugins which already does this, but the process is different. Plugin 1 used for auto liking the current page/post/custom post type. Plugin 2 used for liking a facebook fan page. But both plugin displays an modal to those user's who are logged-in to their facebook account. The modal shows just an image, and a close button. So there's only action available to close the modal. And when user click on the close button, BANG !! user just like the url or fan page. If you are interested, i can give you a demo and price is fixed $50 for those two plugin. Thanks.