Effective branding of yourself or your book online takes repeated exposure to your target audiences. And continually tweeting "Buy my book" on Twitter or writing this same thing for your Facebook updates is not an effective way to build a relationship with people who might then buy your book. If anything, constantly "pushing" your book can turn people off.
How do you get repeated online positive exposure without blatant advertising? You use your non-fiction or fiction book in the service of a good cause or to help someone else.
Here are examples of how this works:
Case A: A new website for women has recently launched, and the power behind the website wants members to write 500-word personal essays and submit these essays to the site. You offer you book as a prize for a winner to be selected by random.org from the women who submit an essay. The website creator is thrilled to have a prize that fits the interests of her members and you're thrilled that your book is going to be promoted as the prize.
Case B: There's an online campaign to raise funds for a good cause, such as breast cancer. But the campaign organizer is concerned, as the fundraising deadline nears, that not enough money has been raised. You offer a copy of your book to each of the top five donors (again picked by random.org). The fundraiser organizer announces this prize to spur higher donations and you're again thrilled that your book is promoted as the prize.
Case C: You're on a virtual book tour and a blogger who will review your book tells you that offering a free copy of the book encourages comments (incoming links for her blog) because to be eligible to win a person must enter a comment. You're thrilled that the contest will go on for two weeks after your book is reviewed, keeping your book's name in front of the blogger's audience for two whole weeks.
Now obviously you have to be willing to give out free copies of your book whenever such a cross-promotional opportunity presents itself to you. But these free giveaways can get more positive exposure than if you were to give a free copy to a reviewer who never gets around to reading your book. Or give a copy to a reviewer who does "read" your book and then writes a "review" that could have been written by reading the back cover.
In addition, one of the unexpected things that can happen when a blogger gives away your book is that the person who wins often may be someone with his/her own blog. Then that person reviews your book, and you offer a giveaway book for that person's blog contest. And that leads to .....