Authors write. They write a LOT. Have you ever been on a forum by authors, for authors? Talk about hard to keep up...
Impossible to keep up and still write books, actually. I've been pulling back on forum involvement, somewhat. Besides, it's not fellow authors that I want to buy my books (they're a loyal lot - "you buy mine, I'll buy yours" - but we can all go broke playing that game).
There are ten thousand author blogs, book blogs, book review blogs, writing blogs... (Ironically, I had one of the first writing-related Web sites in 1994, but so many people jumped on that bandwagon and obviously had more time to do it well than I did that I decided to go back to the "Hello, world, it's all about ME" approach I'd deliberately avoided in creating a site about writing.)
But I digress. I think it can be better to get involved in forums that are NOT focused on your topic (think "a whole bunch of SEO 'experts' all talking to themselves, but not to their clients") - get involved in forums where your skills and talents are in demand, and start a dialogue there. Mommy blogger forums are great for me, as a children's book author. It's all about synergy. While I wouldn't mind having guest posts from other writers, I'd rather have guest posts from parents, kids who like to read, librarians, people (especially men) who didn't like to read when they were kids, teachers...