Go here:
http://developers.facebook.com/docs/.../plugins/like/
To get the code for the Like Button from Facebook.
Follow instructions and copy the code it gives you and paste it into one of your templates, you probably want to put it in your header or navbar if you want to like your entire forum.
There are a couple of mods here on vbulletin.org to add the facebook like button to each thread or post, search "facebook" in 3.8 mods if you are interested.