Find your Target Audience
Find readers of certain genres by searching for groups on Facebook or hashtags on twitter related to the theme or genre of your book. This is where you fans hide out!
Get active! Join these groups, interact and chat with members, share information, read the posts and make comments. Make sure you read the group rules and follow them. Some groups will not allow any promotional posts. I'm a member of over 200 groups on Facebook, and every day I see authors abuse the rules and get booted off.
If you can't promote your book in a group, don't be discouraged from joining. In discussing your mutual love of "x" genre, you might make new connections with readers who follow you back and discover you're an author, possibly leading to a sale and a new fan.
For groups that allow some promotion, don't bombard their wall with advertising, because the members might get offended at the intrusion of some stranger. Introduce yourself. Ask questions. Contribute first. Build relationships. This builds trust with members of the group. Then, when the time is right, simply mention you've written a book. Remember: you're their friend, not an obnoxious author looking to get paid. Don't do the hard sell.
Here are a few groups you might want to join on Facebook and some hashtags to use on Twitter:
If there are no groups or hashtags on your topic, why not create your own? In my book, The Silver Strand, there is a secret magic school inside the hollow earth: a land known as Agartha. I created a group for this called Inner Earth - Agartha, and readers joined. I haven't had to do any promotion of the group. We share information related to this world and its inhabitants. If you decide to follow this route, don't forget to use the 1:7 rule when mentioning your book!
Multiple Genre Authors
If you write multiple genres, like romance and children's books, use the 'create lists' function on Twitter to separate your readers, or create separate pages on Facebook, and only share information relevant to those groups. You don't want to be sharing kinky posts to your followers who buy your children's books!
Manage Your Time Properly
Lastly, try to schedule in a half hour spot for social media engagement with your existing readers to build your following. Social media can be addictive, so limit your time on these sites, as your spare time is better spent on writing new books.
What are you waiting for? You've marinated long enough - time to get cooking on your new social media tactics!