Your fans will definitely contribute your marketing, but you need to keep them interested with new exciting posts, images, videos, stories, etc. They will always like and share your posts, just you need to socialize with them.
It is depends on you Facebook Fan Page. How you maintain the Page and also depends on Fans of that Page. If your Facebook Fans are real, good and regular reader than you get a lot benefit. But instead of this your Fans are not real means you get it from Software or any other Like exchange community than it not beneficial for you.