I've been following Laurie King since I joined Facebook--admittedly, that's not a long time--and she's an awesome example of an author with a strong Net presence. This article by Jeff Widmer (which she links on FB) analyses her activity. For any writer looking to use social media--and who isn't?--it's worth checking out.
My analysis? It's about community. And what starts a community? In this instance, great books that engage the readers' imaginations. But in terms of social media, it's also the fact Laurie King creates conversations. People don't just drop by, they're lured into participating.