I want to add user points whenever user like the page for Facebook. How can I do this?
First, you need to have your users login to your site using Facebook API. There are many modules for it, two I know and seen used are:


Facebook Connect
Facebook OAuth


That's the only real way to know "he" on Facebook is "he" on your site.

Then, Facebook will not push it's data to you. So you have 2 cases:


Button "like this page on Facebook" on your page - you can detect it being clicked and make a rule for it, as  Patrick Kenny told in his comment.
Like clicked directly on Facebook - now that's hard. It was discussed on stackoverflow with some apparently good results, and there are tutorials avaliable.


Last but not least - you need to make sure you grant this bonus only once. If not, users could fabricate points by simply repeatedly liking-unliking your page.