I've often written about the need for a "platform" for non-fiction writers, meaning how much of a following or at least access to potential readers you have. This might be in the form of an e-bulletin, or your friends on Facebook, or audiences of regular workshops or lectures you give, or press contacts.
However, Victoria Strauss of the Writer Beware Blogs says for fiction writers it's not as big an element:
"The importance of platform, unfortunately, isn't a myth. But it's much less of an issue for fiction than for nonfiction, and if you're an aspiring fiction author, a marketable manuscript is still a lot more important than how many followers you have on Twitter. Over the past few years, most of the fiction writers I'm acquainted with who've found first publication have had little or nothing in the way of platform (or previous publishing credits)."
This comment is part of a longer post called "Getting Published is Not a Crap Shoot," in which she emphasizes that although of course there is an element of luck or chance, there are also many factors under your control that help determine whether or not your book gets published.
I agree with that, of course. If you want to get ideas for this, have a look at the posts in the category on this blog called "marketing your book or other writing." Also check out the two chapters on marketing in my book, "Your Writing Coach" (available from Amazon and other booksellers).