It's not a bad idea, but what is the net result? A few million writers and would-be writers trying to get reader attention, creating an amorphous buzz in which it is difficult to stand out. For some interesting thoughts along the same lines, see this post at Mashable.com.
The online element should make up less than half of your promotional effort. Sure, it's easy to do, and sure, all the books and bloggers say you'll have a bestseller if you will just post and twit until your fingers are numb. Unfortunately, the truth is that it is possible to post (or copy and paste) 500,000 words of promo and still not sell 1,000 books.
Stephen King, the late Octavia Butler, Janet Evanovich, Harold Evans, and so many other novelists and non-fiction writers didn't and do not now conduct campaigns to sell books. Yes, their names often sell their books, and some of them use the Web to promote, but they didn't get where they are today because of the Web. They got started before the Web existed; ergo, social networking or social marketing is not a requirement for a bestseller. If posting on hundreds or thousands of Web sites will make a book a bestseller, why doesn't every author have bestsellers?
Some things catch on, and some don't. That's why you need a multi-pronged approach. And the Web may actually be less effective than most folks imagine, simply because there are so many people trying to get attention. When radio was new, it was possible to buy ads and sell anything almost automatically--to millions. Then thousands of merchants and manufacturers were advertising, and each advertiser got proportinally less attention. The same thing happened with television, and is happening on the Web. There are still a few unique venues whose fans buy anything promoted--television programs like Oprah and Lost, which have enormous cult-like followings--but just being on television doesn't guarantee you'll sell. There's more to it than just being there.
In the same way, getting exposure the Web at large is not like being mentioned on Oprah; if you're lucky, or have a brand name, or hit at the right time and place, and the planets align in a certain pattern, Web promotions can work. But there's no guarantee, so extend your book promotion beyond the Web.
As for how you do that ... see future postings.