Kristen Lamb is the best writer on social media for writers that I've found in two years of researching the subject.
In her best-selling book We Are Not Alone she tells writers how to use social media to build a circle of relationships. These relationships are the basis of the "platform" that publishers and agents now want writers to have.
I can't say enough good about her book. She demystifies social media, tells writers how to succeed, and she does it with laugh-out-loud humor. Buy. Read. Use.
Kristen Lamb's blog: Good idea to subscribe to her blog posts.
We Are Not Alone: If you are a writer, independent film maker, or artist, you should stop what you are doing, buy this book and spend the next two days to reading it. It is that important.