The dreaded word- platform.
How important is it, really? I'm not going into that portion of the argument today. Maybe another day, but not today.
"For many writers marketing is almost a dirty word--an ugly truth that must be dealt with in order to make money as a writer." (Novel Writing a Writer's Digest Yearbook publication from sometime over the summer, page 60.)
Several social networking sites exist which can help us build a platform, connect with an audience, and interact with our peers. And it's not just for non-fiction writers. The same magazine I quoted above also had a feature on why novelists need platforms. But there are different ways fiction writers can build a platform. You build it through critique groups, memberships in professional organizations, interactions in the local literary community, named connections that you could use discreetly to your advantage, and the stuff you write about.
There are many social networking sites to join and utilize in building your platform, be it fiction or non.