I've been thinking about this a lot, too. I think there are a few things you can do. You have to give people things on Facebook that they don't get on your site.
We post preview images and covers and occasionally direct people to new web-comics we find. We also post quotes from upcoming issues and the like, in addition to the standard "HAY WE UPDATED YAY!" posts.
But really I don't think FB likes is a great indicator of how you're doing. We've been growing our hit count from 100/day to 250+/day over the past month but only garnered 4 new likes.