It's all about community.
If I could put that in blinky lights here (easily), I would. It's the only thing about social networking you, as a writer, need to remember.
Readers don't blogstalk you so they can read about your release. They don't like your Facebook page because they want to keep an eye on each of the four announcements a day you make about your June title. They don't follow you on Twitter to see whether you're going to tell them about the ebook version...all day.
In fact, if you're using social networking only to promote your book, you're doing it wrong.
The online sociosphere, from blogs and microblogs to LinkedIn and e-mail lists, is inundated with people trying to sell us stuff we don't want. Spammers are everywhere, and many authors are on the verge of getting their own spammy label. Even if you have permission to promote yourself or your book on an e-mail loop or on Twitter...should you?
People gather on social networking sites to network socially, not see ads and self-promo all day. They gather to share conversation, to share ideas, to share images.
But what to share? To start: everything but your book.
When you, the author, decide to jump into Pinterest, for example, because it's a great place to put fun images related to your book, do the rest of Pinterest a favor and create a few non-book related boards. I'm really not that interested in what you think your heroine looks like. I'll care for a second, but then I'll dump that image and go back to the one I had in my head. Likewise, setting photos are cool. But again, they're unnecessary, and I'm not going to follow you on Pinterest if my only option is the board about your book.
Instead, share DIY projects you do in your spare time. Share movies. Share books (not yours) that you love to read. Share funny images you find. Share interesting blog posts. Share things that matter and don't feel like an elaborate ruse to get me to buy your book.
Share yourself. Bits and pieces of you that will draw your reader community closer. Things that will help them find common interests and common ground. Form relationships, not an audience. Your book is for your audience. Your social networking presence is for your community.
Authors who neglect their communities and who treat social networking sites like they're personal billboards to pimp books will see their promotional efforts fail.
Rule of thumb: If you've already mentioned your book today, wait a week before you mention it again unless there's huge, amazing news your readers will want to know about. Also, don't disappear between these mentions. Stick around and connect with others.
Authors who connect with their community, who share the bits of their public lives (via their public personas, a post for another day) with the rest of the online world, are different. Their readers will discover them. Their readers will come to them organically, and when those authors respond to their tweets, thank them for their comments, and spread the author love (aka pay it forward, share community, cross-pollinate, give an author you admire some toys in your playroom), the readers become fans.
I'll give an example. In my almost 20 years hanging out with RWA crowds, I've come across many authors whose books I used to enjoy...until I met them. I've also come across authors who've earned a lifelong fan because of how awesome they were. One example is Catherine Coulter. I adored her historical novels back in the day, but I just didn't gel with her contemporary voice when she switched over. However, I buy her books as gifts for others all the time because one day, she offered me -- someone she'd met two hours earlier -- the chance to send her my WIP so she could give me feedback. I never took her up on it because it was way too generous and my WIP sucked way too hard, but I've never forgotten her kindness. Now, go buy her books because she's amazing!
Translate this attitude--this willingness to reach out, be accessible (by whatever standards you you have or need), be friendly, be part of a community--into the online world, and your readers will become fans who attract more readers.
Remember it's about community.
Remember that nobody is on Twitter to see you pimp your book. They're there to get to know you. Leave the pimping for your website, your bio, and your tiny e-mail signature line. Let your personality advertise you, and readers will look for your book.