Be Interesting And Interactive
One of the great thins about social media is that you get to chat with your potential fans. Watch and see what your friends and fans are saying on the social media of your choice. Make comments. Post something funny that is relevant (or not) to your book. Keep the spam low, but if you do have a new release, a special book sale, or something worth mentioning, such as an award, do tell your fans. They'll want to know, because you've built up a rapport that enables you to market to them. Many would be glad to hear about your upcoming book and where they may be able to purchase it. A warning though Once you post about it, don't continue to repost the same thing. You can subtly remind them with a photo of the latest cover on your fan page's cover or profile page.